Throughout antiquity in the ancient Near East, astronomy, astrology and myth were inseparable. Due to lack of scientific knowledge, these ancients tried to account for the natural world by creating gods and goddess. One of these gods was Mithra, a Persian solar messiah.
Those who worshiped Mithra celebrated his birthday and annual resurrection from the netherworld on December 25th. Around the time that Christ was said to have been born, the state religion of Rome was Mithraism. Early Christians in an attempt to attract converts to their religion adopted many mithraic customs, among these blood sacrifice, communion, baptism, and a dying/resurrected savior that bore the burdens of all mankind. In order for the sins of a follower of Mithra to be washed away, they had to be bathed in the blood of a sacrificed bull. It is also interesting to note, that Mithra had twelve followers or helpers, those being the twelve signs of the Zodiac.
It is for these reasons that Pope Julius I, in an attempt to eradicate the "pagan" worship of Mithra and Sol Invictus, decreed that Christ's birthday would be observed on December 25th.
Here are some terms for you to research:
Yalda, Mithra, Sol Invictus, Zodiac, tauroctony, mystery religions, amanita muscaria, vesica piscis, Zoroastrianism, Yuletide, Saturnalia, Precession of the Equinoxes.