It is not uncommon to hear jokes about persons calling a parish to ask what time Midnight Mass will be at. Such jokes may seem funny, but the question of the time for Midnight Mass and the common answer betray a problem. There are four Masses for Christmas: Vigil Mass, Mass at Midnight, Mass at Dawn, and Mass During the Day. Each Mass has different readings and different prayers. Midnight Mass might not actually be at midnight and, although the correct prayers might be used, all of the Christmas Masses of a particular parish might just have the same readings so that the priest only has to prepare one homily.
So, when one phones in to find out what time Midnight Mass is, he might also want to ask if it is actually Midnight Mass that will be celebrated at midnight.