The 2024 National Trust diary, a copy of which I have, lists St Brigid's Day, Shrove Tuesday, Ash Wednesday (which is also St Valentine's), St David's Day, St Patrick's Day (and the public holiday in Ireland the next day), Palm Sunday, Maundy Thursday, Good Friday, Easter Sunday, Easter Monday, St George's Day, Halloween, All Saints' Day, St Andrew's Day, Christmas Eve, Christmas Day, Boxing Day (which is also St Stephen's Day). I think this is a reasonably representative selection of Christian festivals. The non-Christian religious festivals are also listed, as are the secular Bank Holidays.
The business about the calendar for volunteers - and for their guidance only - has been wildly exaggerated.
Incidentally, at the AGM we were all asked to stand, if we were able, to observe the two minutes' Silence, as it was Armistice Day. We also saw a brief film showing a field full of silhouetted soldiers and poppies commemorating WWI.