From a law professor:

Every time I cover Lee v. Weisman (which ruled prayer at public school graduations unconstitutional), I get a chuckle out of this Souter/Stevens/O’Connor concurrence:
“But religious invocations in Thanksgiving Day addresses and the like, rarely noticed, ignored without effort, conveyed over an impersonal medium, and directed at no one in particular, inhabit a pallid zone worlds apart from official prayers delivered to a captive audience of public school students and their families.”
So, if a Thanksgiving Day Proclamation is given, and no one is there
to hear it, is it unconstitutional?