He prefers to be called X (short for Xmas, duh), and is one of the coolest kids at Holiday Island. Christmas fled Holiday Island, fed up with being "forced to spread love and joy 24 hours a day", but returned rather than let President's Day run the place. If he's not at Holiday Island, the real-world event of Christmas will quickly cease to exist (this didn't bother Daria much).
In personality, he was a pushy jerk who was happy to freeload off Daria.
His main source of food appears to be Christmas-time baked goods.