Julian Day may refer to:

• Julian day, a continuous sequential count of days since the beginning of the Julian Period on Sunday, January 1, 4713 BC
• Julian C. Day (born 1952), American corporate executive
• Julian R. Day, author, computer project manager and charity fundraiser
• Calendar Man, a DC Comics supervillain, whose real name is Julian Gregory Day

Julian day is the continuous count of days since the beginning of the Julian Period used primarily by astronomers.