Dude…
What special day is 9 months before the Summer Solstice?
It's a sex a death cult. They all get together and fuck on their holy days so that their children are born when the stars are aligned.
The solstice isn't fixed and alternates between the 20th and 21st.
If you don't think Astrology is important to their "Ancient Religion" you've got a lot of learning to do.