72. Christmastide runs from 1st Vespers of Christmas to January 13 inclusive.
Within this time are included:
a) the season of Christmas, which runs from 1st Vespers of Christmas to None of January 5 inclusive;
b) the season of Epiphany, which runs from 1st Vespers of the Epiphany of our Lord to January 13 inclusive.
I Vespers happens in the evening
§ 3. VESPERS BY THE ORDINARY, WHO WILL SING MASS THE NEXT DAY
This is the case of greatest solemnity. It is supposed that the whole celebration of a feast is one thing, beginning at the first vespers the evening before.
P 216 Fortescue, Ceremonies …,Second Edition . 1919 New Impression . 1920
a footnote mentions Christmas as an occasion of this.
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