Collop Monday is part of the Catholic celebration of Lent. Variously known as Shrove Monday, Rose Monday, Merry Monday or Hail Monday, this holiday starts the week preceeding Lent. (It is followed by Shrove Tuesday and Ash Wednesday.)
The name Collop Monday is derived from the name of the breakfast dish traditionally eaten on this day. Slices of meat, primarily bacon, which would have been salted and dried for the winter were called collops, and on this day were served with fried eggs on top of them.
According to Wikipedia: "The word shrove is the past tense of the English verb shrive, which means to obtain absolution for one's sins by way of Confession and doing penance." For this reason, the week preceeding the penance of Lent was known as shrovetide. This gave the Monday during the week before Lent the alternate name of Shrove Monday.
Shrove Monday (and Tuesday following) was a time for self-reflection about wrongs a person has committed during the previous year, and ways that those wrongs can be righted.