internet bunny when this happens to me my treatment providers call it depression. with most people it happens during special occasions like day after christmas, or christmas day, day after easter or the day of easter...

my treatment providers say its because of all the excitement leading up to the event causes the body to produce endorphins\epinephrine to keep up with all the stress and activity needed leading up to the event. but then like a sugar high the body cycles back down with the realization that theres no more work to be done and the event will soon be over/ or is all over crash.

my suggestion if this continues to bother you contact your treatment provider. sometimes stabilizing agents like antidepressants can prevent event crashes like this.