Can you believe that the first Borderlands game came out in October of 2009? I can't either as I mostly ejected it from my memory, but subsequent follow-ups like Borderlands 2 and 3 ensured that I'd never forget this series.
Gearbox is banking on that nostalgia hard with a new "Anniversary Celebration" event, which will span over a month, taking place in Borderlands 3. The first one kicked off today at 9AM PT titled "Boss Week," and enhances legendary drop rates for specific bosses, which you can find below. That'll run until October 8 at 8:59 AM PT, at which point the "Rare Spawn Hunt" will take over, followed by Show Me the Eridium on October 15, then Mayhem on Twitch on October 22, then finally Spooky Sunrise on October 29 through November 4, when the event ends.
More details on all the subsequent events are "coming soon," but suffice to say they all seem loot or item-based ("Show Me the Eridium" will net you more Eridium, and so on). I mean, for some people it's an extra excuse to fire it up: for the rest of you still actively playing it (even with the specter of Destiny 2 Shadowkeep looming), it's just plain extra.
Borderlands Anniversary [Gearbox]