Monero Timeline - A timeline of key developments in the Monero history.

Bitcoin Whitepaper 2008 - October 31

Bitcoin's whitepaper was released in 2008, providing the conceptualisation for Bitcoin.
Bitcoin Genesis Block Mined 2009 - Jan 3

Satoshi runs Bitcoin on his computer and mines the first (genesis) block. This begins Bitcoin, which Monero would later seek to improve upon.
Hal Finney tweets about adding privacy to Bitcoin Jan 21, 2009

Hal Finney, an early Bitcoin dev, and candidate for the pseudonym Satoshi Nakamoto, tweets "Looking at ways to add more anonymity to bitcoin". Just 18 days after Bitcoin had begun.
Satoshi discusses Ring Signatures & Stealth Addresses August 13, 2010

On the BitcoinTalk forums, Satoshi discusses early precursors to Ring Signatures and Stealth Addresses. These became 2 of the 3 key pillars for privacy on Monero. See binaryFate talking more about the forum post here.
Cryptonote Whitepaper 2014 - March

Nicolas van Saberhagen's whitepaper improved on the Bitcoin protocol. It included "untraceable transactions" through ring signatures, an ASIC resistant mining algorithm and a smooth emission curve. Detective work suggests the author(s) faked the release date, and it was really released in March 2014.
Bytecoin 2014 - March 3

Bytecoin launches, and is the first coin to implement the Cryptonote protocol - see Github commit. Unfortunately it had at least 1 large flaw - an 82% premine.
BitMonero 2014 - April 18

Bytecoin is forked by "thankful_for_today", and BitMonero is created, without Bytecoin's pre-mine. See BitcoinTalk thread.
Monero 2014 - April 18

BitMonero is forked away from thankful_for_today, after he failed to collaborate with the community, and is renamed to Monero. See BitcoinTalk thread. The genesis block is dated April 18.
Initial Core Team Members 2014 - April

The core team has evolved over the years, but at beginning it was composed of tacotime, eizh, smooth, fluffypony, othe, davidlatapie and NoodleDoodle (source).
First Exchange Listing 2014 - May

The first exchange for cryptonote based coins is created (cryptonote.exchange.to), and Monero is listed. Liquidity baby! (source).
MRO becomes XMR 2014 - June

Monero's currency abbreviation began as "MRO", but this is the ISO 4217 code for Mauritania's currency, so switched to using "XMR" instead (source).
Mnemonic Seeds Added 2014 - June

A feature we all take for granted now, mnemonic seeds, wasn't present in the first cryptonote implementations. Until added, you had to back up the inidividual keys - source.
Moneromooo's first BitcoinTalk Post 2014 - June 18

Prolific Monero developer Moneromooo makes his first post on the BitcoinTalk forums about mining Monero and setting up a mining pool.
LMDB Database Used 2014 - August

Monero moves from storing the blockchain as a flat file in RAM, to the ACID compliant LMDB database. This was a big move for 2 reasons; 1) 6 years later LMDB is still being used 2) it spurred LMDB's creator, hyc, to join the development team.
Moneromooo's First Commit 2014 - October

The Monero GitHub repository has ~250 contributors, with the most consistently active being Moneromooo. The 1st of his nearly 3,000 commits to the project began in October 2014.
XMR.to Launches 2015 - February

XMR.to launches on BitcoinTalk. Providing a way to pay for goods that accept Bitcoin, using Monero, in order to preserve your privacy.
Tail Emission Added to Code 2015 - March

An important characteristic of Monero is its tail emission of 0.6 XMR per 2 minute block, which came into effect in 2022. It was envisioned early on, with tacotime discussing it on the BitcoinTalk forums on April 24 (link). However, the exact details weren't finalized and added to code until March 2015 (source).
Min. Ring Size >= 3 2016 - March

The Hydrogen Helix v0.9.0 release was full of updates, including introducing a minimum ring size of 3 and increasing the block time from 1 to 2 minutes. These changes went live at the March hard fork.
First GUI Released 2016 - December

Until this point Monero was command-line interface only. Launching a GUI version was a big step towards more mainstream adoption. See Github release.
RingCT Added 2017 - January

Confidential Transactions are a way to hide the amount being transacted. Originally discussed and developed on by Bitcoin developers Adam Back and Greg Maxwell, they were later adapted for Monero by Shen Noether here. Released as part of Wolfram Warptangent v0.10.0.
Monerujo Launches 2017 - August

Monero's first mobile wallet, Monerujo, is released on Android by developer m2049r.
LocalMonero Launched 2017 - August

LocalMonero was launched (source) by co-founder Alex in 2017. It's gone on to be a popular platform for peer-to-peer trades.
Min. Ring Size >= 5 2017 - September

The Helium Hydra v.0.11 release sets the minimum ring size to 5, bans duplicate ring members and enforces ringCT.
Cake Wallet iOS Launch 2018 - January

The first genuine (non-scam) iOS wallet is released, with the source code later made openly available.
Min. Ring Size >= 7 2018 - March

The Lithium Luna v.0.12 release increased the minimum ring size from 5 to 7, which increases transaction privacy.
First HW Wallet Support 2018 - March

The first ever hardware wallet support for Monero was added in the Lithium Luna v.0.12 release. Supporting the popular Ledger Nano S. Support for Trezor didn't come for another year.
CryptoNight v1 Update 2018 - March

Cryptonight was tweaked in the Lithium Luna release to prevent DoS attacks by ASICs.
Bulletproofs 2018 - October

Bulletproofs are introduced in v0.13.0, dropping the median transaction size by 83%, and significantly reducing transaction fees.
CryptoNight v2 Update 2018 - October

Another update to the CryptoNight algorithm is made to deter ASICs, released with Beryllium Bullet v0.13.0
Min. Ring Size >= 11 2018 - October

Each time the ring size increases, the difficulty in linking transactions increases. Release v0.13.0.2 set the default ring size to 11.
New Forum Funding System 2019 - March

The new and improved Forum Funding System (FFS) is completed, and rebranded to the Community Crowdfunding System (CCS) - more details here.
Blockchain Pruning 2019 - July

This blockchain pruning update in v0.14.1.0 made it possible "prune" the blockchain to 1/8 its original file size. Particularly aiding small devices like mobile phones to run their own node.
CryptoNight v3 Update 2019 - July

After a hard fork in October 2018 to block ASICs, they quickly appeared again on the network. CryptoNightR (CNv3) was created and deployed in v0.14.0 to mitigate this. Although developers were aware it would only be a temporary fix.
Trezor Support 2019 - July

The Trezor hardware wallet (Model T) supports Monero for the first time, introduced with the v.14.1 update.
RandomX 2019 - November

Patching the CryptoNight protocol to prevent ASICs had run its course, and a new solution was needed. Developers hyc, tevador + more architected RandomX, a mining algorithm that could utilize the full CPU instruction set, and was heavy on RAM requirements. This ambitious project was launched with v0.15.0
Fluffypony steps back 2019 - December

Core team member, and lead maintainer fluffypony takes a step back from the project, with Snipa taking over as lead maintainer. Fluffypony's intent with the change is to reduce the project's dependency on him (increasing decentralisation), and to free up more time to work on the merge-mined digital asset project; Tari.
Dandelion++ 2020 - May

Broadcasting transactions to the network can allow hostile nodes to associate your IP to the transaction. Dandelion++ makes it significantly harder for nodes to make this association. It went live in v0.16.0.0.
Added support for Tor/I2P Nodes 2020 - May

Until this point, you could send a transaction to a node over Tor/I2P, but then the node was limited to broadcasting it over clearnet (not Tor/I2P). vtnerd's CCS work, deployed in v0.16.0.0, has made it possible for nodes to broadcast transactions over Tor/I2P. Currently it's a manual process to setup, which you can read more about here.
XMR daily supply dips below BTC May 2020

During May 2020, the Bitcoin block reward halved from 1800 BTC per day to 900. It was around this time that the daily supply of Monero then dipped below BTC, and will stay lower for the next 8 years, until the 2028 Bitcoin halving.
CLSAG 2020 - October

As part of the v0.17.0.0 October hard fork, the new ring signature construction CLSAG replaces MLSAG. It reduces a typical 2 in, 2 out transaction by 25% in size, and decreases their verification time by 10% - more info here.
An Atomic Swap Implementation Funded 2020 - October

Atomic swaps between Monero and Bitcoin will allow exchange without an intermediary third-party. Joel Gugger et al. had their CCS proposal funded (2,727 XMR) within 48 hours. See whitepaper for implementation details. Current estimates for completion are Q4 2021.
XMR.to Shuts Down 2021 - Feb 1

For 5 years XMR.to provided a reliable method of making Bitcoin payments using Monero. Due to growing regulatory pressure they decided to shut down, rather than compromise on their core values. It was a popular service in the community, and hence deserves a mention.
P2Pool First Released 2021 - Oct 5

Monero P2Pool is a peer-to-peer mining pool developed by sech1 (press release). It combines the advantages of pool and solo mining; you still fully control your Monero node and what it mines, but you get frequent payouts like on a regular pool.
MAGIC Fund Launched 2021 - Oct 5

The MAGIC fund is an alternative to the CCS funding system. It is registered as a 501(c)(3) public foundation, with full tax deductibility of donations as allowable by law, regardless of asset (eg: USD, BTC, XMR, NFTs, stock). More here.
Tail Emission Begins 2022 - June 9

On June 9, 2022 (source) the variable block reward reached its bottom, and the tail emission kicked in. From this point forwards 432 XMR are released per day as block rewards (0.6 XMR per 2 minute block) - in perpetuity. The intention is that this will provide a permanent incentive for miners to continue mining. Unlike in Bitcoin, where the block reward will keep halving lower and lower, thus requiring a "fee market" to pay for mining.
Fluorine Fermi Hard Fork 2022 - August 13

The Fluorine Fermi hard fork introduced some updates, including Bulletproofs+, view tags, higher minimum transaction fees (discussed in issue 70) and a more secure multisig.
tx_extra size limit added 2023 - March 18

PR #8733 was made to limit the maximum size of the tx_extra field to 1060 bytes. With a rough consensus that Monero should focus on being digital cash, rather than another NFT blockchain. In the time since this PR was initially opened Mordinals (inspired by Bitcoin Ordinals) was becoming popular, and Jpeg images were getting added to the blockchain.
Local Monero Shutdown 2024 - May 7

Afer almost 7 years of operation, Local Monero, a popular peer to peer marketplace for buying and selling Monero, chose to shut down. They cited a combination of internal and external factors forced the decision.
Haveno Reto Launched 2024 - May 14

An instance of the Haveno Project, named Haveno Reto was officially launched. Originally a fork of the Bisq peer-to-peer marketplace, it allows users to buy and sell Monero for fiat and other tokens.

Sources:

Bitcoin whitepaper - link
Cryptonote whitepaper - link
Brandon Goodell's review of the CryptoNote whitepaper - link
Bytecoin first appearance on Github link
BitMonero launched on BitcoinTalk - see thread
Monero launched on BitcoinTalk - see thread
Cryptonote Forums - Monero thread - it appears this domain is offline, so am linking to WayBackMachine snapshot - link
XMR.to launches on BitcoinTalk - see thread
GetMonero Roadmap - link
Monerodocs - Monero Technical Specs - link
Ring Signature Confidential Transactions for Monero by Shen Noether - link
How did BitMonero become Monero? - StackExchange thread link
David Latapie's write-up - the history of how Monero got started - link
Blowing the lid off the CryptoNote/Bytecoin scam (with the exception of Monero) by rethink-your-strategy - link
Monerujo launched - see Github
CakeWallet launched, see Reddit announcement, with source code later made available on Github
Reddit: "How much did bulletproofs actually reduce the tx size?" - link
GetMonero: "Monero Adds Blockchain Pruning and Improves Transaction Efficiency" - link
GitHub ReadMe on CryptoNightR - link
Monero Outreach: Monero’s RandomX (archived version) - link
Get Monero: An update from the Core Team on some technical responsibilities - link
Monero Outreach: Dandelion++ for Monero - link
GetMonero - CLSAG audit results - link