I went digging through youtube, old posts, and shitty websites, along with my memories, and here you go! Hopefully these are accurate.
2013 — Unoffical Tumblr event “Mishapocalypse” happened, an online flash mob event wherein which Tumblrinas change their profile pictures to a specific picture of Misha Collins of Supernatural fame.
2014 — Users were given the option to get Tumblr Pro for free, and those who accepted were given top hats on their icons. Prompty after this, @staff announced that “Everyone with a top hat is now marked for account deletion. This is the only way we could destroy this horrible website. Happy April Fools day.”
2015 — The “Executive Suite 2016 Productivity Edition” essentially changed Tumblr into office software, allowing spreadsheets for memes, calculators that gave incorrect answers, and Coppy. Who gave “helpful tips”.
2016 — Tumblr voted to select the “new lizard king”, from Rick, Debrah, Mop and Wretched Tooth. However, more famously, an edited @staff post reads “for april fools we’re deleting this entire site sayonara you weeaboo shits”.
2017 — The Tumblr Horse Game was a feature that, when clicked, took users to a game wherein you had to collect shit from a pixelated horse. If you failed to do so, the horse died.
2018 — The answer to Bitcoin, is Tumblcoin! A parody of crytocurency.
2019 — Tumblr Memories, in which Tumbeasts were set loose. Remember them? The mascot from 2011, for service interruption announcements.
2020 — There was seemingly no prank this year. This was COVID-19.
2021 — Tumblr released “non-fungible tumblcryptids”, a parody of NFTs. There was a supposedly limited amount of them.
2022 — A light switch, when activated, would open up a variety of colourful things on the desktop dashboard, including a “Summon Crab!” button, which would summon a crab when activated. Other buttons made different sounds.
2023 — A feature similar to the Discord reaction function was temporarily added, using basic emojis.
2024 — Every user was given the option to opt-in to the boop o meter, and could boop, super boop, and evil boop other users who also opted in, earning up to three badges by doing so.