Reasoning:

4/20 feels very left-libertarian to me. That is, the people that are more concerned with having access to all kinds of drugs and abortion-on-demand than sound money, property rights and freedom of assembly.

4/19 is, for me, the way better date. That's the day that the American Revolution started (Battle of Lexington and Concord in 1775). That's a much better meme than "OMG weed day!"