I doubt 'solemn' is the word to use here. The Solstice celebration gets pretty solemn as it progresses, and yet it's clearly not the kind of ceremony you're thinking of.

The kind of ritual you seem to be suggesting... the only people (in fact and fiction alike) I can think of who've had rituals like that are brotherhoods of warriors/insurgents.

Which might be what you're aiming for, but I don't think our peaceful circle of philosophers should take up arms against the Enemies of Sanity just yet.