Why celebrate Christmas in December?

Recently I had a friend who was apparently upset with some who had challenged the date of December 25 as the date of Christ's birth, and who used a sermon from a former pastor which "proved" it as the true date. First, let me say that I in no way am against celebrating Christmas and wish to put no damper on anyone's enjoyment of this time of year. However, I am against belittling or putting down anyone who questions man's traditions using Scripture as a basis for their beliefs. Therefore, I am making my belief of Christmas known, based as much as possible on the Bible and what I have learned over the years of Jewish tradition during the time of Christ. Here goes: First, my friend stated that the shepherds were in the fields with their flocks, keeping watch over them because it was the time of year for them to give birth..I agree that December(or thereabouts) would be the time for the flocks to birth. However, shepherds would not have their flocks out in the fields for this to happen, since it would be very difficult for them to protect the newborns from the elements and the predators of that region at that time. They would have them in sheepfolds, where they could be tended easily and protected. Secondly, if we use the account of Luke, we find that Zacharias served in the temple during the 8th course. The priests were divided into 24 courses, each course serving two weeks a year, one week at the time. Using the Jewish religious calendar, in which Nisan is the first month(roughly approximating our April), he would have been serving around the end of our May. After serving in the temple, he goes home to Elizabeth, and she becomes pregnant with John the Baptist. Counting nine months for term delivery, John would have been born sometime around March of the next year. We know the time of Mary's visit, and can calculate that Jesus would have been born about six months after John, which would put His birth around September. Why do I believe this is the true time of Jesus' birth? Well, Scripture tells us that Jesus fulfilled all prophecy and the law. September is the time of the God-ordained feasts of Tabernacles, Trumpets, and Atonement. What better fulfillment of the previous "shadows" of the feasts in the Old Testament than Christ coming to "tabernacle" with us at the time of this feast? The feast of Trumpets- declaring God's chosen to all, and Yom Kippur,atonement, revealing Christ's purpose on this earth..Again, I have no problem with Christians celebrating Christmas, I just am eager for us to know WHY we do things, and to rest upon the Bible for the reasons why we do what we do. After all, it wasn't His birth that was the focal point of His life on earth, it was His work on the cross and His glorious Resurrection which gives us hope and brings Him glory..Merry Christmas!