Fast of the Firstborn

This fast commemorates the fact that the firstborn Jewish males were saved from the plague of the firstborn in Egypt.

When is it observed?
On the day preceding Passover and only by firstborn males.

What does Fast of the Firstborn mean?
Like many of the other events in the Jewish calendar, the English meaning is the literal translation of the Hebrew. Thus in this case, "Ta'anit" means "Fast" and "Bechorim" means "Firstborn". So it comes together nicely as "Fast of the Firstborn".