What is Side piece Day?

Celebrated the day before (February13)or the day after (February15) Valentines day. A day men spend with their side piece as if it was Valentines Day.

I was with my other girl on Side piece Day

Side piece Day - video

February 15th The day men take there side piece(the other girl he is dating on the side) out as if it were Valentines day.

Tommy bought 2 bouquets of Roses One for his girlfriend on Valentines day and one for his side piece on side piece day.