Christmas Day
The festival was the Roman winter solstice and the Germanic festival of Yule. The date was chosen as the most favourable for the celebration of Christ's birth, and many of the ancient Yule customs were amalgamated into the Christian festival. It was also the date of the birth of Mithras.

The modern Christmas has an interesting story. [Note: all dates are given according to the modern Gregorian calendar]
Early Christians had a single festivity: Easter, which was celebrated according the Hebrew religious calendar. Whitsun began to be widely celebrated in the third century, well before Christmas.
The first to celebrate Christ's birthday were the Egyptians and the date was February the 6th (not universaly, see below). This too was a previous festival: the birth of Osiris. If you have any idea of how influential North Africa was to early Christendom (just think of St Augustine) you won't have problems accepting the fact that Christmas was more or less "imposed" by decree in 353 AD under the influence of Carthaginian and Egyptian clergymen. The chosen day was December the 25th for political reasons: it was the birthday of Mithras, often latinized as Sol Invictus.
Mithraism was an immensely popular religion in the Roman Empire: just like Christianity it had universal appeal. Mithras was worshipped by aristocrats, merchants, soldiers and slaves alike.
Aeliogabalus, one of the many Roman emperors more known for their extravagant lifestyles than for their feats of arms or reformations, was an ordained priest of Mithras. Diocletianus and Maximianus both prayed to Mithras as "Protector of the Empire".
"Take control of their holidays and hearts and minds will follow".
The true birthday of Jesus Christ is shrouded in mystery.
Clemens Alexandrinus, the celebrated Father of the Church, wrote in his "Stromata" that some believed that the correct date was April the 9th and others May the 20th. He himself believed the correct date to be November the 17th.