Pre-Lent. “To shrive” means to forgive sins. “Shrove” is the past tense, and a person who has received absolution has been “shriven.”
I haven’t looked up the history, but the timing suggests that people went to Confession in the days before Lent, had a minor blowout on “Shrove Tuesday,” and then did their penance during Lent.
At least they ate all their animal fat/bacon grease. LOL!