Why does iCal forget the calendars? Sometime ago I noticed that iCal lost all my evens and calendars. Luckily I was somehow able to "re"-synchronize it with the Outlook calendar.

Yesterday the same happened. After opening iCal all my events calendars and tasks are deleted. The funny think here is, if it comes closes to an event it still keeps me notifying that the event will happen.

Did someone experience something similar? How can I prevent iCal from forgetting the events and calendars?

I am working with Snow Leopard and iCal 4.0.4 and Office for Mac (but I do not use Outlook itself). Everything is stored locally and not synchronized with any iCloud, Google Calendar or services like that.
Make sure that in your iCal settings all of the boxes about deleting past events are unchecked. 

Also, if you have numerous devices' calendars syncing to your Mac/iCloud, make sure that in each device (i.e. iPad, iPhone etc) under calendar settings the "delete events older than..." are disabled.