Liturgically and traditionally, according to most of the Fathers, Dec. 25th is calculated based on March 25th (the Annunciation & probable conception of Jesus), which in turn is based on the September conception of St. John the Baptist (Sept. 23rd) and his birth (June 24th). However, such calculations, while possibly literal and accurate, are primarily symbolic and liturgical (the daylight after June 24th decreases and the light increases after Jesus’ birth connecting with the Baptist’s words: “He must increase, but I must decrease”).
But while the Fathers proposed different dates for Christ’s nativity, history, liturgy, and scripture do prove that Christmas is not Sol Invictus or Saturnalia dressed up in Christian clothes (not that Christian assimilation of good elements from the natural or pagan world is bad). Jimmy Akin offers an analysis on how difficult it is to claim Dec. 25th as an absolute date. https://jimmyakin.com/…/12/was-jesus-born-december-25th.html