What Is the Day After a Holiday Called?

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Keeping this in consideration, what is the day after something called?

There isn't a standard name for the day after something. Unless that particular day has a name such as boxing day.

Also Know, what is the English word for day after tomorrow? overmorrow. [ oh-ver-mawr-oh, -mor-oh ] the day after tomorrow: I've heard that tomorrow and overmorrow may bring exceptionally high waves.

Keeping this in consideration, why is there no word for the day after tomorrow?

There isn't an official word for day after tomorrow in English. Though there once was a word that was literal translation of the German word übermorgen and the literal translation was overmorrow. You can say it is obsolete now, nonetheless you can use it in informal writings.

Why is the day before called Eve?

My own cursory research reveals that "eve" comes from O.E. æfen "evening", and that the first appearance of "eve" in its current meaning first appearing in the late 13th century [source]. God called the light “day,” and the darkness he called “night.” And there was evening, and there was morning—the first day.