Birthday is the fifth day of the week. Birthday originally had a different name, but Dimwit Flathead changed it when he took the throne (c. 770 GUE). Any earlier records to a name before Birthday have been erased.

Birthday, before the fall of the Empire, was the official celebration of Dimwit's birthday. On this day, which was observed one a week, everyone in the kingdom was required to give the king a present. By the year 883 GUE, it was associated with big sales at U-Mart and J.C. Zorkmids. Even after his death in 789 GUE Dimwit was excessive, still having his birthday celebrated every week. Dimwit's Birthday is only one of the special holidays that occurs throughout the year.

1 Sand Day
2 Mud Day
3 Grues Day
4 Wands Day
5 Birthday
6 Frob Day
7 Star Day

SOURCE(S): Zork Zero, Encyclopedia Frobozzica