According to goarch.org
, it states, "On the Sunday that falls on or immediately after the twenty-sixth of this month, we make commemoration of Saints Joseph, the Betrothed of the Virgin; David, the Prophet and King; and James, the Brother of God. When there is no Sunday within this period, we celebrate this commemoration on the 26th."
I'm just hazarding a guess, but it may be because this year the Sunday after the Nativity also happens to be the Feast of the Circumcision of Our Lord Jesus Christ and St. Basil's Day (Jan 1), and that may supersede the celebration of the aforementioned saints, so they get pushed back to Dec. 26th.