Annunciation and Christmas

I have been wondering lately why the celebration of the Annunciation came to fall on 25 March (in today's civil calendar).

I was looking up things about it, and found that in an internet article about the date it says that Ephraim the Syrian taught that the Annunciation fell on 10 Nisan in the Jewish calendar, which is the day the Passover Lamb is selected.

Still unanswered is the question: where did Ephraim get 10 Nisan?

Here's my theory: Mary herself told Luke.  So he didn't write down that she told him.  That's not important.  What's important is: from whom else would he have found out?

He wrote about a lot of details no other evangelist mentions.

Occam's Razor: the simplest solution is usually the right one.