Yeah, more or less. Spitballing, but it's probably still more related to the Sun. The 25th would've been on of the first days studious Romans could tell the daytime was growing longer.
This whole period in Roman History is extremely cool. Maybe not to live in though lol.
My first rec would be Rod Bennett's The Early Church in Her Own Words. It's a good starting point that grounds you in what the early authors of Christianity were thinking and discussing.