Here's the timeline:
1 Dec The final two states certify their election results to their Governors on this date.
6 Dec The Governors of all states certify official results to Congress. If this deadline is met (all states having resolved all 'issues') there can be no more legal challenges to election results.
14 Dec Electoral College casts votes. This determines the actual outcome of the election.
6 Jan The election results are formally certified in a Joint Session of Congress.
20 Jan Eviction, I mean, Inauguration Day.