Mondasian Christmas

According to the diary of a time traveller, Mondasian Christmas was an annual event on 25 June, (PROSE: Time Traveller's Diary) unlike Christmas on Earth, which was on 25 December. (TV: Voyage of the Damned)

Behind the scenes

 * Technically, Mondas having a festival called "Christmas" can possibly have a lot of implications on Mondas' history: in that — not unlike Earth — Mondas somehow also had the specific course of events involving Jesus and early Christianity that lead up to the creation of Christmas.
 * However, the simplest explanation is that the time traveller in Time Traveller's Diary misunderstood a festival on Mondas as "Christmas", in the same vein many people in the real world misunderstand the Day of the Dead as "Mexican Halloween".