The ancient collect as found in the Roman Missal and in the Anglican Missal:
Almighty and everlasting God, vouchsafe, we beseech thee, to govern us in all our doings in the way of thy good pleasure: that we, faithfully serving thee in the Name of thy well-beloved Son, may worthily abound in all good works. Who liveth and reigneth with thee and the Holy Ghost, ever one God, world without end.
1549 and subsequent Prayer Books
ALMIGHTY God, who hast given us thy only begotten Son to take our nature upon him, and as at this time to be born of a pure virgin; Grant that we being regenerate, and made thy children by adoption and grace, may daily be renewed by thy holy Spirit; through the same our Lord Jesus Christ, who liveth and reigneth with thee and the same Spirit ever, one God, world without end. Amen.
The Medieval pattern was to use several collects, one after the other, a practice still kept in some Missal parishes. Thus both the specific collect for the Sunday and the Christmas collect (as a commemoration) were used. Cranmer, in the interest of simplicity, preferred to use but one Collect, whenever possible. It would appear that, as the Sunday collect seemed a bit overly generic during such a high festival octave, he chose to omit it and use only the Christmas Day Collect. This latter appears to have been his own composition, artfully weaving together the themes of the various Latin collects for the three Masses of Christmas Day.
He came down for us, and took our nature upon Him.
He became man that man might be restored to union with God.
We are adopted as sons and brethren, redeemed, restored, forgiven.
May we be given strength to walk in all the good works to which he has called us.