December 18 is when they had meeting where they decided to plan Jan 6th. I think before that trump likely asked Barr to make DOJ use something illegal to hold power.
BTW: I found it interesting when Barr was discussing trump's documents handling indictment, he said that AG still had some discretion about who special counsel charges and some other things. Then he realized that this applied to Mueller and him so added that that case was "obviously" bogus.