I've now looked at the versions of the Cherry Tree Carols that have the unborn Jesus prophesy his day of birth, as I didn't know that verse. Before, I might have thought that the date referred to Epiphany which, in much earlier time, was a celebration of the birth of Christ, but lost that function to another possible date, that we now known as Christmas.
However, to my delight I encountered both
- On the sixth day of January
- On the fifth day of January
as the date.
That suggests that it is indeed about Old Christmas, which initially was was on the 5th, and is now on the 6th. I guess that supports as well my explanation that in 1800 Old Christmas was shifted, compared to the Georgian Calender.
But it goes further: Before 1752, that birthday would not have been in January at all. It would have been
- On the twenty-fifth of December, my birthday shall be
Except, we don't seem to have that version.
Let's speculate: If there had been such an older version, it could have existed much longer than the 48 years of the Fifth of January version, yet we don't have the former while we apparently do have the latter. The version that survived from the Coventry Mysteries, which were banned at the end of the sixteenth century, doesn't have this birth prophecy. Other variations of the Cherry Tree Carols, even longer ones, usually don't include this part either. Plus, in this part of the carol, the rhyme scheme is often broken (CHERRY TREE CAROL in the DT).
Now, what is the purpose of this verse? It's not really about the cherry tree any more. Instead, we have Christ himself tell us what the date of his birth was.
It's speculation, of course, but it doesn't seem all that unlikely that the part about the date of Christ's birth was inserted after 1752, exactly because of the calender shift: The author(s) have Christ himself tell us that the correct date is Old Christmas.