Here's a good place to start concerning Jesus and John's birthdays:
If you haven't read through "Christ in Egypt" yet, then you may enjoy getting a copy and reading through some of the scholarship Acharya provides concerning the many links between John the Baptist and Anubis. I find them rather interesting myself.
As for issue of John's head being cut off in the allegory, it's pretty well established that we're looking at a reference to the constellation of Aquarius having it's head appearing below the horizon around August 29th as is discussed by Acharya in her works. Here's a link concerning the entire gospel stories allegorical structure and if you click on "John the Baptist's stars" in the month of January when Jesus' ministry begins you can see a few illustrations of the astrological meaning of John's conception, birth, and beheading:
_________________The Jesus Mythicist Creed:
The "Jesus Christ" of the New Testament is a fictional composite of characters, real and mythical. A composite of multiple "people" is no one.The celestial Origins of Religious BeliefZG Part 1Jesus: Hebrew Human or Mythical Messiah?