Coined by the host of Aural Salvation, Fuck Season begins the Friday before Easter and officially ends November 1st, or whenever a person decides to stop observing the "season" by getting into an exclusive relationship.
Generally speaking, people hook up post-halloween (or on Halloween) with people they're going to brave the winter time with. This could include the Halloween parties, but usually includes Thanksgiving, Christmas/Hannukah, New Year's Eve, and Valentine's Day. The most common months for Birthdays range from January through to March (with the highest number being February), so often there's a birthday included in this time period.
After this succession of holidays, many couples have little to celebrate together, and no reminders to appreciate one another, so they often grow apart and break up shortly thereafter. This mass relationship exodus then leads to "fuck season" beginning and carrying through for most of the year, or at least the extended spring/summer seasons.
If you'd prefer a "lighter" term, you can also call this period "Hump Season". Mating season sounds too clinical.
Number One :
"I'm getting ahead on my spring cleaning duties right now"
"cause.... fuck season's coming soon. Gotta get prepped"
Number Two :
"baby... it's not you. It's me"
"yeah. As in it's me that's gotta dump you 'cause fuck season's starting"