Prokeimenon of the day

The prokeimenon of the day is, on a few special days, replaced by a great prokeimenon, which has more verses, similar to the Saturday evening prokeimenon. (The latter is, in fact, a great prokeimenon.) This happens at vespers on the same day as a great feast of the Lord. An exception is when that vespers service happens on a Saturday evening; the Saturday evening prokeimenon is never replaced in this way.