sl/Birthday Clowns Frowns Clowns

Milo spent a great deal talking about his birthday today. Coincidentally, Sidney's birthday is this Tuesday...

Is there any day in the year that isn't someone's birthday?
Has that ever been confirmed?

Milo told me all the important things he's bringing to Sidney's party.
"You gotta bring a smile," I reminded him.
"You can't go to a birthday all frowning."