Ever since Fortnite’s current season debuted in June, there has been the promise of a massive conflict with a group of aliens. Now developer Epic is finally teasing that big event in the form of “Operation: Sky Fire.” The live event will be taking place on September 12th at 4PM ET, and Epic says that players will “join a strike team and sneak aboard the Mothership to deliver IO’s final message to the invading aliens.” It will also be a one-time only thing: if you miss it, you’ll have to catch up on YouTube. As always, Epic is recommending players log in early to get a spot.

The kinds of live events have long been a highlight of the Fortnite experience, dating back to the original rocket launch in 2018. Since then players have witnessed erupting volcanos, battling giants, a flood, the infamous black hole, and participated in a battle with Marvel supervillain Galactus. Epic has even experimented with single-player events. The upcoming alien event has been teased for some time, with a giant mothership hovering quietly over the Fortnite island for months; several locations have also been destroyed by the invasion.

The event will also mark the end of season 7 of Fortnite, which means that if you still have battle pass characters to unlock or gold bars to spend, you’ll want to do it before September 12th. Outside of aliens, this season has featured some notable Fortnite moments, including an Ariana Grande concert series and the addition of a controversial mode called Impostors.