Safe Harbor Day. That was yesterday, and I think all states met it except Wisconsin, which is dealing with a lawsuit. If I understand correctly (and I am not totally sure I do), Congress created this deadline to guarantee that the electoral slates of any state that certified its election results by this day would not be subject to challenge when Congress officially counts the electoral votes in January.
If so, that would mean Republican challenges in the vote-counting session of Congress to the electoral slates from Michigan, Pennsylvania, and Georgia should automatically be invalid, but they could legitimately challenge the slate from Wisconsin. As both the House and Senate separately have to agree to reject a particular state's slate of electors, we can be certain that's not going to happen.
But I'm curious to see if some of the nuttier members of the GOP do in fact try to challenge the MI, PA, and GA slates, and whether procedurally their challenges will simply be ruled out of order or not.
My guess is they will indeed challenge those states -- Wisconsin at a minimum, maybe others. If they get shut down, it just supports their narrative. If they get to have a debate, they can spout their ideas on a national stage, even if it's a forgone conclusion that no challenge will be upheld by both houses. So we're in for a stupid show, whether we like it or not.