Here is an excerpt of this article from Bible History Daily:
Clement writes: “There are those who have determined not only the year of our Lord’s birth, but also the day; and they say that it took place in the 28th year of Augustus, and in the 25th day of [the Egyptian month] Pachon [May 20 in our calendar] … Further, others say that He was born on the 24th or 25th of Pharmuthi [April 20 or 21].”
fn. Clement, Stromateis 1.21.145.
Get a head start on your greed and gluttony and start planning for mid-April!