Tauman hit the nail on the head. Ever since 2013 it's been something of a tradition to have a major worldwide roleplaying event on the site every year around April 1st. They traditionally start on April 1st NZ/AU time (so March 31st if you live in America), and continue for around two days or so, though there's usually a short 'prelude phase' on the night before to cue everyone in.
To join in all you really need to do is be on the site during the event. The main story thread will usually be marked out with something along the lines of 'APRIL 1ST - MAIN THREAD' in the title, and once you've found that you just need to start posting as one or more characters. Traditionally the events have the mod team taking on the role of the 'bad guys' trying to take over the orbital, while the members RP Tau characters defending it, but any member is welcome to play a protagonist or antagonist character regardless of rank - it's entirely up to you which side you're on. Likewise, if you want to be a Tau character, you're welcome to write about any kind, from a terrified Por'la civilian to a battle-hardened Shas'O and anything in between, regardless of what rank or caste you are on the forum.
There is something of a loose continuity between the events. The plot of one year's story can often reference a previous year's events (the 2014 event, for example, featured a force of Inquisitors from the Imperium seizing the orbital to investigate the Chaos attack that happened the year before), and several long-time posters have their own special characters that they carry over between events. Don't let that deter you however, everyone is welcome to join in, and the more the merrier.
For reference, a complete list of April 1st event threads are as follows:
- 2013: Embrace Change
. "24 hours of Tzeentchian horror, the clock is ticking". The event that started it all. As the 6th edition Tau release, the first new wave of content and models for the Tau since 4th edition, draws closer, the sheer volume of hopeful emotions regarding coming changes opens a warp rift on the orbital. Drawn to the feelings of hope and change, the legions of Tzeentch pour out and lay waste to the Orbital (as well as a contingent of Slaaneshi Noise Marines).
Originally planned to be a simple visual gag, a number of members began to post 'in-character' comments on the main rumours thread discussing the then-upcoming 6th edition codex, and from there the whole thing snowballed into the famous epic fondly regarded by a number of members.
- 2014: The Emperor Calls (Turn Not Away, Lest Ye Be Judged!)
. In the aftermath of the epic battle in Embrace Change
, Imperial servants of the Inqusition arrive and place the orbital on lockdown while they investigate for signs of heresy (and the Arch-Magos accompanying them secretly ransacks the station for advanced technology). But the Tau aren't so keen on the idea of being pruged...
Following on from the success of 2013's event, the mod team put together a second one, notable for its plotline which was created almost entirely on-the-fly.
- 2015: A Miniature Vesa Taps Your Hoof
. After three years of careful plotting, an ancient conspiracy comes to fruition and the Necrons take over the orbital by subverting all advanced technology onboard. Without the aid of their high-tech battlesuits and wargear, the Tau defenders will have to triumph using their wits alone...
The third April 1st event, this time with a more centrally driven plotline (and a notable low-tech vs high tech theme).
- 2016: Artemis Trellium's Tradestop
. The ATT defenders have fought off Daemonic hordes, Imperial fanatics and Necrontyr monstrosities, but now they must face... salesmen? Using dubious legal pretences, a profiteering Rouge Trader arrives and announces he has purchased the ATT orbital wholesale. As the station is converted into a trading platform, the Tau are unsure of how to react, but some suspect that Artemis Trellium may not be as legitimate as he seems...
The last April 1st event on the old forum.
- 2017: The Holographic Principle
. Having secretly monitored the ATT Tau since Embrace Change
, an alliance of Craftworld Eldar makes an attack on the planet the station orbits, seeking to destroy it from below in order to safeguard their plans for the future. But the ATT Tau have other ideas...
The first April 1st event after the forum reboot. Unfortunately an unexpected repercussion from the changeover was that the new forum architecture is much harder to modify (especially with a limited timeframe), making the traditional RP event too difficult to manage. Thus, this event featured more emphasis on collective storytelling instead of directly roleplaying characters. An in-universe justification of sorts was an enemy attacking the planet beneath the orbital rather than the orbital itself.
- 2018: coming soon. If you're interested, get the word out and make your support known. The more encouragement us mods get, the better.
There's also a few smaller supplementary threads around some of the later events, but they're too buried among the subforums for me to find them all.