Here's something you don't see every day ... 2 movie stars riding bare-ass nekkid on a scooter -- but when ya do, it sure makes for a good day.
Sebastian Stan -- who plays Bucky Barnes, the Winter Soldier in the 'Avengers' flicks -- and Irish actress Denise Gough busted out their birthday suits Wednesday morning and hopped on the scooter to cruise through Athens. It's not a dare or a drunken escapade ... they're just filming for the upcoming movie, "Monday."
Check out the gallery ... Sebastian and Denise seem totally at ease riding in the nude. So, they're either great actors or kinda freaky. Or both. They're definitely not shy.
We don't know what the movie's plot is all about, but suddenly ... we LOVE Mondays.