Gwyn wrote:Does this mean that Alma Redemptoris gives way to Ave Regina on the Monday after The Baptism of Our Lord? Or does this happen from February 2nd?
By tradition, after February 2nd. The General Instruction of the Divine Office (no 92) says that, at the end of Compline "one of the antiphons of the Blessed Virgin Mary is said. In Eastertide this is always Regina Caeli..." but it doesn't otherwise specify which is said (hopefully sung) when.
FrGareth wrote:Now, consider the Sacred Triduum. Is any part of it in Lent? Is any part of it in Eastertide? How much of Easter Sunday is Triduum and how much is in Eastertide. Is it in both?
The Sacred Triduum lasts from Evening Mass/Prayer on Maundy Thursday to the end of Evening Prayer II of Easter Sunday. The latter is very specific: "The end of the Easter Triduum" - and oddly this directive is printed before Compline of Easter Sunday, so that is when Eastertide begins.
If you have, exceptionally, a Mass of the Last Supper earlier in the day on Maundy Thursday, this would count as part of the Triduum. But the Chrism Mass, even if it is celebrated on Maundy Thursday, is still within Lent.