Calendar Man (real name: Julian Gregory Day) is a DC Comics supervillain. His main enemy is Batman. He first appeared in Detective Comics #259 (September 1958). He was created by Bill Finger.

Calendar Man is fascinated by dates and calendars and usually always revolves his crimes around what day of the week it is or in the case of the day having a special anniversary he will plan his crime around that day. He also wears different costumes which correspond to the crime of the day that he is commiting though he does have a main costume which has various numbers (meant to represent days sprouting from the shoulders).

He is notorious amongst both heroes and villains alike for being something of a joke as nobody really takes him seriously (including the fans themselves and Calendar Man has long been criticised for it).

He is probably best known for his appearance in the classic Year One era story The Long Halloween, where he was a Hannibal Lecter like figure.

In recent years comic book writers have tried to make Calendar Man much more of a greater threat to Batman as told in the third issue of the 80 Page giant Batman Special Editions (July 2000) titled All the Deadly Days.