I have never understood where the idea that Jesus' birth was "really" in April (or whenever) came from. I've heard people say that it has something to do with the presence of shepherds in the tale, or some recorded celestial event that might have been the "star in the East", or the census taken by Augustus Caesar. None of these make sense to me, both for practical and historical reasons - shepherds herd sheep more-or-less throughout the grazing months, there is no specific, single astronomical event we can point to and say definitively "that's the star of Bethlehem" (moreover, stars and comets, in case you hadn't noticed, seldom hang over a single town), and a Roman census would typically take months or years to complete.
Here's the main thing though - there is no historical record of the birth of Jesus, period. There's only the Gospels, and they are demonstrably bad history. In fact, it's open to question whether there really ever was a historical Jesus at all!
In the most concrete terms, this much is certain: there was never a person named "Jesus" who was born a Jew and claimed to be the Messiah of Jewish prophecy... because "Jesus" is a Greek name, not a Hebrew one (his real name might have been something like "Yeshua"). That seems trivial, but it points to something important that must be remembered when reading the Gospels - the four books were all written by Greeks who lived and wrote long after Jesus' death, spoke no Hebrew or Aramaic, and whose understanding of Jewish scripture relied on oral transmission and translations of dubious quality*. In my opinion, these early Greek-speaking Christians are much like modern New Agers who pick up bits and pieces of ancient or foreign religions, astrology, quantum mechanics, et cetera, contort these source materials to fit their own ideas, and then try to form it all into a semi-coherent whole.
So the gospel versions of Jesus' birth are straight-up Origin Story, a naked attempt to cram multiple poorly-understood Old Testament prophecies into a single narrative in order to establish the Messiah's bona fides. The mistakes give the game away: Luke, for example, knows that Jesus was called "the Nazarene", and that he preached in Galilee, but he also is aware of a prophecy that the Messiah must be born in Bethlehem (possibly via reading Matthew) - so he writes of a Roman census which for some reason required people to return to their birthplaces to be counted. The census is real - he's explicitly referring to the Census of Quirinius (http://en.wikipedia.org/wiki/Census_of_Quirinius), an infamous event to the Jews of the time - but the requirement to return to one's home town is 100% fiction. Why would the Romans care in which tiny village the peasants of some far-flung colony happened to be born, as opposed to where they lived and worked today? Furthermore, the timing contradicts Matthew's account - we know that King Herod died nine years prior to the census, and after the census Judea had no king at all, being under direct Roman rule. Herod, of course, is credited by Matthew with killing all the children of Bethlehem in an attempt to murder the infant King of the Jews - an event which no other records of the period (including the other three gospels) bother to mention.
The problems only get worse from there. Many people are aware that the origin of the "virgin birth" story probably stems from the mistranslation of a word in Isaiah that might better have been rendered "a young woman". That's just scratching the surface, though - turns out it's Chinese Whispers all the way down! "Bethlehem Ephratah" in the book of Micah is not a place, it is a tribe or clan (named after its patriarch). "Nazareth" was not a place, either**... A "Nazarene" was a member of a Jewish sect, similar to the Essenes whom you may have heard of. Matthew's account of Herod's slaughter and Jesus' family's flight into Egypt are also presented as fulfilling prophecy, but neither is actually a prophesied event - the phrase "I called my son out of Egypt" in Hosea is obviously referring to Moses' Exodus, and the slaughter of children in Jeremiah refers to the Babylonian captivity (see also: Psalm 137).
In short, the gospel accounts of Christmas are pure mythmaking, and without them, there is nothing to hang any speculation about Jesus' real birthdate on whatsoever - which, when you think about it, makes sense. If Jesus was a real person and had anything like the biography depicted in the Bible, he was more or less an ordinary, if outspoken, carpenter for his whole life prior to starting his ministry sometime in his thirties. Why would anybody have thought to record the details of his birth, at the time? And if signs and portents had made his divinity so obvious, why does nobody pay the slightest attention to him again for at least thirty years after the Magi showed up bearing gifts?
*FWIW, there are many serious Biblical scholars, including some skeptics, who argue that John may really have been one of the original disciples. However John, like Mark, doesn't talk about Jesus' birth, or his (human) life prior to his ministry at all.
**At least not at the time. A town of Nazareth first appears in the historical record around 300-400CE, and is a Christian settlement.
