Sometimes my game starts to crash at the turn of the month. It has happened in both of my latest games, at some point. At the end of some random month it'll just start crashing every time you reach that date. In my game as Duchy of Leinster it always happened at the end of October 1088. I tried Veldmaarschalk's advice, opening my savegame in Notepad and setting my startdate a couple of days ahead, but then it would just start crashing at the turn of the month after.
This is the error message I would get:
######## EXCEPTION: 0xC0000005 at address: 0x0044BC53: ACCESS VIOLATION read attempt to address 0x00000180
12/24/09 09:30:47
StackWalk(): GetLastError = 487
Well, I just found a way around it.
I load the game up in a different realm than my own (maybe of of my vassals). Then let the month turn. Save. Then load as my own realm again. Then I can continue playing as before.
The thing is, that if you're a king, the AI will automatically hand out all your Duchy titles (which I like to keep to myself). So make sure you hand them out beforehand, so they go to vassals you can trust.
Another thing that works some times, is lessening your demesne size. This crash often happens to me when I have a large demesne. Giving out a few titles might do the trick, if you're lucky.