How to keep notified events in calendar?

Since the Mountain Lion upgrade (maybe I didn't noticed it before), iCal has a nasty habit of reminding me of an event and then erasing it from the calendar before the event actually happens. I forgot my friend's birthday yesterday because the notification popped up Thursday, I okay'ed it and the event was gone on Friday when I looked at my calendar (having forgotten already).

How do I get it to leave the notified event in the calendar?