That's because Dec. 8 is known as the safe harbor deadline. Under the Electoral Count Act of 1887, Congress must count the electoral votes from states that chose their electors and resolve any legal disputes over the choice by the act's deadline, which is six days before the electors meet to vote.
Electors seated between Dec. 8 and Dec. 14 — the date when electors are required to meet in their states and cast their ballots for president — can still vote, but they could theoretically be challenged by Congress.
The safe harbor deadline, however, is something of a guarantee. The federal law says a state qualifies for the safe harbor protection if it has resolved "any controversy or contest concerning the appointment of all or any of the electors." State court cases remain active in six states that certified Biden the winner — Arizona, Georgia, Michigan, Minnesota, Nevada and Wisconsin. So they may not be able to claim the protection unless their cases are resolved by Tuesday.