I feel compelled to repost my comments from another thread Now, in particular this has to do with Christmas, but the thought remains. (The OP in the other thread was the husband.)
First, let me say that a lot of things might have "pagan" origins, but - in the modern day and age they have completely lost that meaning. Just to play Devil's advocate for a moment - does she call the days of the week something else, given that for example, Thursday is named after Thor and Friday is named after Frigga? (Norse pagan gods.)
As for a discussion about the origins of the Christmas tree, like anything of folkloric tradition, its origin is up for debate, and historians have a variety of theories as to the original purpose behind the practice.
Open warning - I'm a Master's level History major graduate, so - this might get boring, boggy, and technical.
Yes - there is a commonly held belief that the Christmas tree began as some unknown "pre-Christian" ceremony of unknown purpose, however, evidence for the practice of a Christmas tree as we know it (AKA decorating it, and putting it out specifically for Christmas time) doesn't show up in historical writings into well into the Middle Ages. None of the accounts of pre-Christian times (AKA writings from Romans or Greeks about Celts or Germanics peoples) specifically mention decorating a tree, particularly an evergreen as a celebration, as most of the focus is the native interest in oak trees. Popular Victorian-era psuedo-historical books like "The Golden Bough" over-emphasized the importance of tree worship to Druid-led Celtic groups, so it's not a big leap that the two things have been linked together - Christmas trees and "pagans", even if there's little to no historical evidence to support that theory.
Instead, the Christmas tree was thought to have been a practice began by a shipping Guild, who used the tree as a way to store prizes (quite often food) for their annual holiday party. Concurrently, trees were also used for medieval mystery plays (theatrical representations of Biblical stories) to represent the tree in the Garden of Eden, and as such were decorated with apples, and wafers. These plays were performed on Christmas Eve, to remind parishioners on why Christ's coming was neccessary - the Fall of Man, and sin, and so - the tree, covered in apples served as a reminder. Over time, the apples gave way to shiny red ornaments and the wafers into homemade cookies, and candles (which sounds really dangerous) were added to represent the Light Christ brought to a dark world.
Christmas trees for home decoration were most popular in German Lutheran areas, where Christmas trees were used to decorate the home and show denomination allegiance, as Catholic homes preferred to decorate with a creche (AKA a manger scene) instead. Hessian soldiers brought to Canada and America by the British introduced the practice to North America.
Ultimately, the practice became worldwide, and was adopted by cultures and people the world over. Each country/culture has its own symbols as to what generally goes on the tree, when the tree should be put up or taken down, etc. Ultimately, these days the Christmas tree serves a purpose even in families that are not Christian, or are non-practicing - it serves as a family ceremony of yearly remembrance, decorating with ornaments passed down over the generations, and purchased over a life time.
As for how to compromise with her on this, perhaps you should explain that last part - assuming that is the reason you enjoy the practice, that it reminds you of family, that it brings up good memories, that it is a family tradition. Explain that you are willing to also decorate with more religious symbology as well ( a manger scene?) but you are hoping she can understand why this is important to you.
This whole Christmas tree as pagan thing is off, there's 0% historical evidence that "pagans" even utilized, decorated, or had "Yule trees" in the sense we imagine. Also the whole "Christmas trees as secular" thing is off too, as noted above.
As to the Halloween thing, yes, that's inherently pagan, and inherently commercial. So is Valentine's Day if you want to get into it.
I do think that if the OP's husband feels so strongly about it, he should put his money where his mouth is, and send the kids to religious school. But expecting them to ostracize themselves and make themselves out for ridicule because of what makes him uncomfortable while attending public school is unfair on them. School is hard enough socially without having to constantly say "My dad doesn't like it."
And what's the fine line here about how much his beliefs can dictate his family's life before it's gone overboard? I mean, if he sets up the red tent in the backyard, and starts expecting his wife to sleep out there during her period, is that okay too?