The Sunday preceding Shrove Tuesday; the next day is called Hall' Monday, and Shrove Tuesday eve is called Hall' Night. The Tuesday is also called Pancake Day, and the day preceding Callop Monday, from the special foods popularly prepared for those days. All three were days of merrymaking. Hall' or Halle is a contraction of Hallow or Haloghe, meaning holy or festival.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894