Pentecost

From the website Full Homely Divinity:

Pentecost is, as the name denotes, the fiftieth day (Greek: pentēkostē) of Easter, the last day of the Great Fifty Days. Although it does, like Ascension Day, commemorate its own major event in the history of salvation (i.e., the coming of the Holy Spirit), it cannot stand alone. Properly speaking, it is not the beginning of a new season of the Church year. Rather, it is the end of a season, the last hurrah, as it were, of the celebration of the Resurrection of Jesus Christ.

Nevertheless, Pentecost (or Whitsunday as it has been known in England) has traditionally been treated as a new feast. In effect, the pre-Vatican II Roman Catholic Church observed an octave of the feast, appointing propers that focused on the work of the Holy Spirit for the weekdays between Pentecost and the following Sunday. (Curiously, though, the week was still described as being part of Eastertide.) This was also reflected in older Anglican practice where The Book of Common Prayer used to provide propers for the Monday and Tuesday in Whitsun Week. Furthermore, the Ember Days which were observed at the end of the week also focused implicitly on the work of the Holy Spirit. Now, however, Ember Days have become a liturgical footnote and most modern Anglican use no longer explicitly connects the week to the feast day that begins it. On the day after Pentecost, ferial (non-festal) time begins. The numbered Sundays after Pentecost do not constitute a season of Pentecost. Rather, Pentecost, the last day of the previous season, is just a convenient marker to begin counting from. Oddly enough, if Pentecost has a season at all, it consists of the nine days that precede it. Both biblically and liturgically, the novena which is properly Ascensiontide is a period of preparation for Pentecost.