Christianity borrows from so many other traditions, to be sure. One of the “traditions,” that Christianity borrows from is nature itself. As nature, give us “seasons” of “four seasons” of summer, autumn, winter, and spring,” The Christian tradition is replete with various seasons as well such as “Lent,” “Pentecost,” “Ordinary Time,” “Easter,” and of course, “Christmas” which the Christian calendar is currently in right now.
While nature’s four seasons are fixed to the sun’s aperture relative to the Earth’s hemispheres and tropics, Christianity’s seasons correspond to the phases of the moon in the spring, and are fixed to the Gregorian calander for fall and winter feasts and preparatory days.
Advent, Christmas, and Epiphany are always “fixed dates,” while Lent, Easter, and Pentecost are the “movable feasts.”