The date chosen is statutory, not constitutional.
The Court may disregard the statutory date set for tomorrow and proceed with this emergency.
The Electoral College is now on notice that whatever business they conduct between now and January 20, 2021 can be set aside.
Whether the Court acts on this or not, the events of December 18, 2020 will start.
The authority to set the date chosen is constitutional. Just as the constitution says that Electors are to be selected an the manner that the state legislatures determine (not by a court,) the U.S. Constitution says that the date the Electors meet is to be set by the U.S. Legislature, not by the courts.