Firstly, all Christians celebrate the Feast of the Nativity on the 25th of December. Work it out!
This is actually a misnomer, a_o, and will, to an extent, depend on which calender one is working on. Some Orthodox believers celebrate early in what we would call January. Whilst we were working in nepal, the Nepalese church was seriously considering moving Christmas to the end of October/early November, so as to coincide with what is the main Nepalese cultural festival.
Secondly, the 25th of December is not a date "merely plucked from the air". As I said on "The Trinity" thread:
"Both the scriptures and creation mystically point to midwinter. ...
Except, of course, for those passages that suggest that it occurred at a different time of year. And therein lies the problem: there is no Biblical passage that provides a definitive timing of the event. However, extra-Biblical records - such the Roman records concerning the census and scientific records concerning the comet that is likely the 'star' - place the event sometime in early to mid-autumn - late September/early October.
... We know that St. John the Baptist was born six months before our Lord and in the Gospel he says of Christ "He must increase, but I must decrease". Creation also testifies to this, for the Church celebrates the Nativity of St. John the Baptist on midsummer after which the Sun decreases, and the Nativity of our Lord Jesus Christ on midwinter after which the Sun increases. Both the liturgical calendar and the cosmos are in harmony and of that we should not be surprised for creation speaks of him through whom all things were made."
http://www.religionethics.co.uk/index.php?topic=9512.msg563030#msg563030
As I understand it from what I have read, the celebration of Jesus' birth didn't become common until the 3rd or 4th century AD, and most saints' days became popular in the 4th or 5th, so dating Christmas by referring to a saint's day is the wrong way round.