Karl Smallwood, at Today I Found Out:
According to McDonald’s executives, approximately 23% of the Filet-O-Fish sandwiches sold by the chain each year are sold during Lent. While the exact rules vary somewhat from region to region, in general, Catholics traditionally abstain from eating most meats on Fridays and Saturdays during this period. In some regions, the requirement is even that members of the Church avoid non-seafood meat on every single Friday of the year.