The days after Palm Sunday are “privileged Ferias” and no feast whatsoever is allowed to have the office. I and II Class Doubles are transferred and all other feasts must be commemorated at Vespers & Lauds (no comm. lesson at Matins).
From the beginning of the Sacred Triduum, until (and including) the Tuesday after Easter Sunday any feasts falling on any of those days are ignored completely and omitted for that year.
The Sacred Triduum has special, very ancient, features for which pls read the Note on p. C288 and pay special attention to the red directions throughout the Office. To say them correctly it is best to take note of these special directions before saying the Office. The special directions given at the point where Psalms are broken up are there to allow you to reduce the burden of the Offices for the following days. Remember that there are no Absolutions, Benedictions, or introductions for the Lessons during the Sacred Triduum. The addition of Psalm 51 (anciently counted as Psalm 50) is a very ancient feature of the Offices. In both the Roman and Sarum uses Psalm 51 was thus said toward the end of each Hour of Prayer.
Fr. Gregory +