How about ingame holidays (which correspond to the real life calender )?.
Some basic ideas:
On Halloween, ghosts and some "demons" or so are going around - or the Darkness expends significantly for that day- or you can craft Jack-O-laterns out of a pumpkin and a torch, or Boston is decorated with Jack-O-Laterns
Thanksgiving/ Harvest festival: Bringing food to the church - and purity of the food increases in the church
Walpurgis Night (30th of April towards 1st May): Witches get some bonus/access to a certain spell/skill improvement when they meet up around a fire in a minimum number- Darkness also Expends
Eve of St. George: small blue flames can be seen in Marshes, Grasland and when you dig there you can find some coins or so