Christmas (IPA: /kr?sm?s/), also referred to as Christmas Day or Christmastide, is an annual holiday celebrated on December 25 that marks and honors the birth of Jesus of Nazareth. The birth of Jesus, which is the basis for the anno Domini system of dating, is thought to have occurred between 7 and 2 BC. December 25 is not thought to be Jesus’ actual date of birth, and the date may have been chosen to correspond with either a Roman festival, or with the winter solstice.

Shab-e Yald?  or Shab-e Chelleh is an Iranian festival originally celebrated on the Northern Hemisphere’s longest night of the year, that is, on the eve of the Winter Solstice.

The 13th century Iranian poet Sa’di wrote in his Bustan: “The true morning will not come, until the Yalda Night is gone”. Following the Iranian calendar reform of 1925, which pegged some seasonal events to specific days of the calendar, Yalda came to be celebrated on the night before and including the first day of the tenth month (Dey). Subject to seasonal drift, this day may sometimes fall a day before or a day after the actual Winter Solstice.

Following the fall of the Sassanid Empire and the subsequent rise of Islam, the religious significance of the event was lost, and like other Zoroastrian festivals Yalda became a social occasion when family and close friends would get together. Nonetheless, the obligatory serving of fresh fruit during mid-winter is reminiscent of the ancient customs of invoking the divinities to request protection of the winter crop.

Yalda today
Families continue to hold traditional gatherings on Yalda.[citation needed] Iranian radio and television offer special programmes on that night of Yalda.
Watermelons are placed on the Korsi, a traditional piece of furniture similar to a very short table, around which the family sit on the ground. On it, a blanket made of wool filling is thrown, people leave their legs under the blanket. Inside the korsi, heat is generated by means of coal, electricity or gas heaters. Youth hold Yalda parties.

Reference : http://en.wikipedia.org/wiki/Yalda