Tuesday, December 8 is the Electoral College "safe harbor" deadline; if a state chooses its electors and certifies its results by that date, it receives some protections against challenges to them. The electors then cast their votes “the first Monday after the second Wednesday in December,” which, this year, is December 14. Federal law requires the states to send in their electoral votes to Congress by December 23. Those votes are then counted in a joint session at 1 p.m. ET on January 6, 2021, and, per the 20th Amendment to the Constitution, the previous terms of office for president and vice president end on January 20 at noon.
Learn more about this process, and broader debates about the Electoral College, in our resources linked below.
From the National Constitution Center: