The "history.com editors" using a phrase like "it is commonly believed" doesn't feel like a citation to me. My understanding is that the date is derived from the date of the Annunciation.

Expecting a formally reviewed citation in response to a vague three word query in an informal conversation on a forum wholly unrelated to specialization in that topic is a bit silly, but if you do bother to do the legwork and find a citation for either what I linked or your own notion, it may be of interest to other readers.