Valentine's Hangover
Valentine's hangover has two definitions.

1. Something bad has happened on Valentine's day, and therefore you are depressed for a period of time afterwards. You have a Valentine's Hangover.

2. Something good has happened on Valentine's Day, so you are ecstatic for a period of time afterwards. You have a Valentine's Hangover.
1. Molly: Why are you so sad?
Daniel: It was Valentine's day yesterday, and my girlfriend dumped me. I have a Valentines's Hangover.

2. Daniel: Why are you so happy?
Molly: It was Valentine's Day yesterday, and my boyfriend proposed to me! I have a Valentine's Hangover.