Did you hear about the guy that went to a nudist colony? The first day was his hardest.
Nudist Colony Visit
Explanation
Explain Bear
Listen up, buttercup. This joke is all about wordplay. See, a "nudist colony" is a place where people walk around with their birthday suits on. When the joke says the "first day was his hardest," it's implying the dude got a boner! It's funny because it's a cheeky double entendre.