Christianity is just another one in a long line of Resurrected Sun King Cults. From the Orphic Mysteries, and the rebirth of Osiris as Horus, or the Persian Mithras, Chaldean's Bel, Adonis of the Phoenicians or the Greek Apollo.
There as so many common themes, it's almost like a trope, or formula. Central to all of them, is the Solar Trinity. A Holy Trinity, or triune Godhead is not particular to Christianity. Far from it. The Persians, Greeks, Phoenicians, Hindus, Babylonians, Romans, Egyptians, and Norse all had their Trinities too. In every instance, it represented a threefold aspect of the Godhead.
For the Egyptians, the Sun manifested as a Trinity. It had it's diurnal day and night cycle, and it's annual procession through the the year, visiting 12 houses, each of 30 days. These are it's calender aspects.
But also, there's another pattern here, called "The procession of the equinoxes". In this, the Sun retrogrades through the zodiac, at the rate of one degree, every 72 years.
So basically, the Sun is said to allegorically conquer each of the signs as it passes through them. In Taurus, the Sun became a Bull, and was worshipped as such by the Egyptians and the Cretans, as Apis. By the Chaldeans and Assyrians as Baal, or Bel.
In Sagittarius, the Sun becomes an archer. In Leo, it was Heracles, slayer of the Numean Lion. In Pisces, it was a Fish, worshipped as such by the Philistines as Dagon, and the Hindus as Vishnu.
All of the most Ancient Gods and Pantheons resolve themselves in the Solar Fire. And each Priesthood held up The Sun, as Godhead. The Supreme Deity is always the Divine Light of the Sun, personified.
The Greeks believed that many of their Gods and Heros partook either of the visible, or the invisible attributes of the Sun. (Zeus, Apollo Uranus, Bacchus, Dionysos, Hephaestis, Heracles, Jason, Odysseus, et al )
The Norse had Baldur as a Solar Deity, and Odin Skyfather is associated with the Sun. especially signified by his single eye.
In Egypt, Ra, Horus, Ammon, Osiris, Anubis, and Hermes all had Solar association, Isis was Mother of the New Sun with the birth of The Crowned and Conquering Child God, Horus. He would wait his time to avenge his Father, and battle Typhon/Set as the year goes on, culminating in the Death of the Sun at the Winter Solstice. Then, on the third day after the Solstice, the Dawn get's a little earlier. The New Sun, (New Son) is clearly seen to have been resurrected on the 25th of December or Yule. Incidentally, the same date the Christians chose for their resurrected New Son
The three days for which the dawn appears at the same time, 22nd, 23rd, and 24th December, these are the three days Jesus/Mithras/Horus/ he's said to be "nailed to the tree/ or cross. The Cross, in this sense means the cross of the Soslti and Equinoxes, as they quarter the wheel of the Year, at 90 degree angles.
The constellation of Virgo can be seen to actually give birth to the New Sun at dawn on the 25th December, the Dawn rising just at the point on the horizon where VIrgo, as Venus rises up to start her funny little horn shaped passage across the sky again. Which, having seen with my own eyes, I can attest to it's exactitude. Also, the source of the Virgin Birth of the Christian catechism. I think Mithras' mother was some kind of celestial Virgin too, and Isis was The Virgin of the World. because she gave birth to Horus in a very complicated, but specifically "No Penis Involved" way.
So to be brief (
The Romans had these days marked "13 days before the calends of September"and adopted by Paul,as the Catholic feast of the Assumption. The Ancient Greeks and and Romans also mark that day, as the start of the Assumption of Astrea/Astarte/Ishtar/Eaostra.
Those three days are really important to be kept separate and aside from thee rest of the year because every fourth year, the 72 degree retrogradial motion of the Sun, demands that an extra day be taken from it, and this is where we get our leap year from. 365.25 days to the Solar year. So, each year gets an extra day for it's quarter, and the fourth year gets a day to make up the last 3 quarter days, into one whole day. (Spread over a four year cycle) That might even be Algebraic, I'n no mathematician, but the calender works like clockwork now.