One of the joys of the liturgy nerd is the changing over of breviaries at the hinges of the liturgical seasons. Today, Pentecost, is one of those days. If you pray the Divine Office with either the typical edition Liturgia Horarum or the American English Liturgy of the Hours, there are five such switches during the year. Curiously, they do not always come on the same day of the week. Sometimes seasons end after Evening Prayer II of a Sunday, such as on the Baptism of the Lord or Pentecost, when in both cases Ordinary Time starts with the Monday following. (Oh, let us mourn for the Pentecost Octave!) Other times the new season begins with Evening Prayer I of a Sunday, as is the case with Advent and the non-seasonal change from one volume of Ordinary Time to the other around the beginning of August. In the case of the beginning of Lent, a switch of volumes is made on Tuesday night or Wednesday morning.
Here's how it breaks down:
IV-I Saturday (Ordinary Time-Advent)
I-III Sunday (Christmas-Ordinary Time)
III-II Tuesday (Ordinary Time-Lent)
II-III Sunday (Easter-Ordinary Time)
III-IV Saturday (Ordinary Time-Ordinary Time)
I consider Sunday to be the "switch day" for seasons beginning on Monday because of the universal permission to anticipate the Office of Readings after Evening Prayer of the previous day.