Game hasn't updated to bring in a Halloween event for this year yet, once the event is ready the game will update and there'll be a contract for a Halloween extraction mission as well as festive decorations in the garage. Once the Halloween event is over, typically a few days after Oct.31, the game will update to remove the event and garage decorations. Whatever festive collectables during the special extraction mission you've picked up will remain in your inventory to be put on display or sold, and special tanks like the EventMaus and LittleWillie666 tanks will remain in your inventory as well.

EventMaus is special in that normally you can only get two Maus tanks maximum from extraction missions, and they don't show up in the scrapyard, but you can always buy parts and hulls from the store. EventMaus is basically a fun way to have extra chances to 'earn' a Maus in the field, and typically are the missions where you find festive collectables.

As for the toggleable event, it just means garage decorations will be toggled when such festive events are up.