The Constitution says nothing about Congress's establishing the date of Halloween.
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 ;)