In Jewish tradition the Evening begins the day. We continue that practice with Sundays
That is a common perception, but is not quite true. If it were, Sunday would end at sundown, and it plainly does not. The day runs from midnight to midnight unless explicitly otherwise. However, the *celebration *of Sundays, Solemnities, and Feasts of the Lord falling on Sunday begins on the preceding evening.
So, on the solemn celebrations enumerated above, Evening Prayer I is said on the evening preceding the feast, Evening Prayer II is said on the feast itself. Should 2 such celebrations occur back-to-back, ie when there appears a conflict between EP II of the day itself and EP I of the day following, EP of the celebration of higher rank is said. If the celebrations are of equal rank, the EP of the day itself (ie EP II) is preferred.
For instance, this year: Christmas has a higher precedence than Sunday, so on Sunday 24-Dec, EP I of Christmas is said rather than EP II of Sunday. In a week, however, Holy Family is a Feast of the Lord, but by its falling on Sunday a Solemnity equal in rank to the Solemnity of Mary, Mother of God, therefore on Sunday 31-Dec, EP II of Holy Family will be said.
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