I'm not counting on having a "special" occasion (e.g. an oral presentation, meeting, class, etc...), but things that happen almost everyday. For the most part, mine is when I go to sleep at night.
This mostly happens during breaks from school/college, since then I have my plate full with work and have to worrry about that. When I'm on Summer Break, for example, every night I dread going to bed and the time that it takes to fall asleep, due to being a time where one is alone with his thoughts. No escape, no distractions, nothing to make you forget them. Just you and your angst, worries and fears, free for them to haunt you until you fall asleep.