That along with the gloomy hex atmosphere makes me wonder if this is a sort of sequel to dark ages in the way that Adventures was to Seaside and Empires is to Prosperity.
I could imagine it as a fixed set of Alchemy: Potions made the game too slow, but you want to have some card that are running outside of the regular coin economy, so players can pursue different progress tracks. The night mechanics opens this path without the problems of a new currency.
My guess is that the night mechanic works similar to Outpost or Mission: After this turn, take another turn, where you can only play night cards. Night action cards would then allow you to curse your opponent, play remaining dead terminals, gain other (night) cards, do funky night stuff, etc.
Hmm, thinking of it... That could be quite some words on the cards. Maybe it is far more simple: It could work like an automatic Journey token. During setup you place the day / night token on its day side , then before the start of your turn you flip it. During daytime the day effects on cards are in play (i.e. Citizen / Werewolf -> Citizen: +1 Card +1 Action). Then night falls and the night effect of the card becomes active (i.e. Citizen / Werewolf -> Werewolf: Opponents discard to 3, +3 Cards).
Anyway, I bet there will we a card that on setup requires you to start at night instead of day.
EDIT: I overlooked the 'which are played after the Buy phase' in the short description from Jay. So I guess the night token is out.