Some festivals, like Christmas Day, happen on the same date every year, while others move around within a range of dates.

The reason why some of the Christian festivals not on the same date each year is because the Christian Calendar grew out of two other Calendars, the Jewish and the Roman. In their distant past, the Jews were a nomadic (wandering) people. As they often travelled at night, the moon was of great importance to them, and they based their calendar on its phases. The first great Christian festivals sprang from Jewish ones. The Christian Church grew and expanded under the Roman Empire which followed a calendar controlled by the sun. When the church began to introduce festivals of its very own, not based on the Jews, they fixed them on dates already in the Roman Calendar. The Christian Calendar is thus a dual one, with 'fixed' feats based on the Roman 'solar' calendar, and 'moveable' ones based on the Jewish 'lunar' calendar.