Actually, his name is Dave. So I asked him why he calls himself Sunday.
“It's a long story.”
“I have time.”
“Well, my ex-wife and I owned a house on a beach in Maryland. Next door was a restaurant/bar. On Sundays, a local aerobatic plane club would practice maneuvers over the water, in front of the bar. It became very popular, and the room at the bar overflowed into our yard. During the week people would point at me and say 'Sunday', meaning that on Sunday they'd be in our yard. The name stuck.”
“Now that your ex has the house, what do they call her?”