Claim: Responsories for Holy Week applies to the last three days: Maundy Thursday, Good Friday and Holy Saturday, which Triduum Sacrum (translation: three days) included Maundy Thursday but not Easter Sunday (before 1955).
Evidence: Responsories for Holy Week (Latin: Responsoria pro hebdomada sancta) are polyphonic settings for the matins responsories, not of the whole of Holy Week, but only of the last three days: Maundy Thursday, Good Friday and Holy Saturday. Also before 1955 the term Triduum Sacrum, which now includes Easter Sunday and takes in only the close of Maundy Thursday, was applied to the whole of Maundy Thursday, including its matins, and excluded Easter Sunday.