On major feast days, and today we celebrate one of the biggest-Pentecost, we have what is called a sequence before the alleluia before the reading of the Gospel. Usually sung, it is a chant or work of poetry connected theologically and biblically to the feast being celebrated. While not all were used liturgically, the sequence became a popular Catholic literary genre in the twelfth and thirteenth centuries with nearly 5000 examples. Most of the sequences were removed, however, after the Council of Trent in the Missal of Pius V in 1570. On Easter and Pentecost the singing or reciting of the sequence is mandatory, while the few other sequences are always optional.