September 21, the Autumn Equinox is, of course, a massively important day in the Christian calendar, one the Four Most Important Days of the Year for Christians.
Just like Easter always fall on the Spring Equinox.
Oh, wait. It *never* falls on the Spring Equinox? Hmm…
Also, there’s no evidence that Christmas was invented to replace the Feast of Sol Invictus. There is evidence, however, that the Feast of Sol Invictus was pumped by a pagan Emperor trying to take over the brand of Roman Christians who had already been celebrating the birth of Jesus “eight days before the kalends of January” for decades previously.
Rosh Hashana (Happy New Year, Jewish readers) is sensibly tied to the agrarian cycles that dominated the lives of a pastoral people. A good time to have a New Year and so the Jewish people do right now. And Christianity certainly incorporates some of that into its own symbolic language. So Easter is tied to Passover (not the equinox except indirectly, because it falls on the first full moon *after* the equinox). And Pentecost was a Jewish harvest feast which God, in his wisdom, made an image of the “firstfruits” of the Church.
But is a real stretch to call Christianity “warmed-over paganism”. What’s extremely notable is that the early Church does not seem to give a fig for pagan cultic practices, feasts, or fasts. Everything is about Jewish or Christian scripture and tradition and that’s it.
So today is the feast of St. Matthew. Nice guy. Great apostle and evangelist. But all the apostles get feasts. There’s nothing about today that sticks out as central or important, because the Church attaches no special significance to solstices and equinoxes per se.