we have the precedent of Congress sanctioning and setting the date of Thanksgiving in 1941, after a century of Presidential meddling.
'sides, from the Preamble: "We the people... ordain and establish this Constitution... in order (to accomplish the following tasks, including) insure domestic tranquility"
Regularizing Halloween in a rational manner falls under that header, IMHO ;)
Oooooh, you're one of the "all unenumerated powers are in the Preamble" crowd. If you weren't the King, I would feel quite negatively about that ...
But aside from that, I'm just a Month and Day person. Christmas, Halloween, Veterans' Day ... What's Thanksgiving doing on the THURSDAY, anyway?