Julian Day (Earth-4678)

"Thirty days hath September, April, June, and November. All the rest have thirty-one... Except for February alone."

- Calendar Man

Julian Day, better known as Calendar Man, is a villain who times his crimes thematically to certain holidays throughout the year. He is obsessed with the quirks of the calendar, which makes every evening on Gotham City a potential day for Calendar Man to kill.

Source

 * https://dc.fandom.com/wiki/Julian_Day_(Arkhamverse)