Not the same as Green Day which is the greatest band ever.
A green day, (which is what the band are named after) is a day where you bunk school, work, or whatever you were meant to be doing and spend the whole day smoking greenie or weed or whatever you feel like calling it today.
Kid 1: Where was you yesterday?
Kid 2: Oh, yeah. I had a green day with my mates from round my area.
Kid 1: And you didnt tell me?