I wouldn't have an issue with celebrating His birth if we could say exactly what day of the year it falls on. There's nothing in Scripture that says one way or the other, so I just don't see the point.
It does not matter to me in the slightest what day of the year Jesus got born, everyone deserves a holiday and a piss up at the end of the year, none of this Christmas in July bullshit.
if you want to be logical about it, well if you don't that's up to you of course, but if you do, i'd go with the 25th of what is now october. (as pointed out, the actual day of the month is indeterminate. the day that is all saints or all hallows, might be logically replaced however) i'd skip the tree, as that came from pre-christian celtic/druidic cultures. those i would celebrate at midwinter's eve however, though christian extremists might wish to forego mention of them, and just have a feast a tree, and maybe a fire on the hearth. (which even rome, had yet to know well, and certainly had never yet at that time, reached their colonies in the middle east) gift giving would still be valid (the zoroastrians who stumbled on the even persuing astronomical phenomina) but avoid making it the a marketing feeding frenzy, which, i'm not sure how you'd do that, the toys kids (and adults) are mostly mass produced and technological in nature. a feast would be good of course, almost any holiday, or any other day is a good excuse for that.