They do these events at least once every year, usually around the anniversary of ffxi. that's also why they give out the anniversary ring (exp bonus), to entice returners to play making leveling easier.
Tho, imho, if your server is anything like Bahamut.... there isn't much to return to anymore.
They killed it off as an "event" and now you can just return when you want. SE stopped delting accounts because no new registrations are coming in. Depite that being said, Fenrir. lol. Return to that POS? Funny.
Doubt they're gonna bring back Anni rings as an event outside of the yearly census.