Multiculturalism can mean a lot of things, but it's generally part of an argument that minority cultural traditions and practices should be given a more equal status alongside majority cultural practices which have long dominated people's lives. This presents challenges to people prefer those majority cultural practices, but sometimes they find a way to make it work for them. One example is the attempt to defend explicitly Christian celebrations of Christmas in a multicultural context.
Catholic League President William Donohue is a major proponent of this tactic and published an advertisement in The New York Times to advance his claim that celebrating Christmas is a way to "celebrate" diversity. Right from the beginning, though, he helps reveal the principle flaw in this position:
The United States is 85 percent Christian, which means we are more Christian than India is Hindu and Israel is Jewish. Moreover, 96% of Americans celebrate Christmas. So why do we have to tippy-toe around the religious meaning of Christmas every December? ... Diversity means respect for the traditions and heritages of all groups, not just hose which have been cherry-picked by the multicultural gurus.
If a particular cultural practice really is part of the lives of such an overwhelming majority of people, then celebrating it isn't "celebrating diversity" and promoting it isnt a way of promoting multiculturalism. On the contrary, it is precisely in contrast to such cultural behemoths that multiculturalism tries to work in order to introduce a bit more diversity and a few more options. This provides non-Christians, atheists and theists, a way to avoid Christmas or observe Christmas in their own ways.
The second major flaw in this argument is the claim, sometimes implicit and sometimes explicit, that the "religious meaning" of Christmas is somehow being suppressed, denied, or otherwise infringed upon. It is suggested that secularists and atheists are infringing on Christians' religious liberties. We see this claim made implicitly above and then again more explicitly here:
There is something sick about Friendship Trees, Winter Solstice Concerts, Holiday Parades and Holly Day Festivals. The neutering of Christmas extends to the banishment of Nativity Scenes from the public square, the expulsion of Baby Jesus from creches not otherwise forbidden, the banning of red and green at school functions, the censoring of "Silent Night" at municipal concerts, etc.
Note that no one is complaining about being forced to erect a "Friendship Tree" in their homes or being in any way being forced change how they personally celebrate Christmas. A person who wants to celebrate a religious Christmas that has all secular or pagan aspects eliminated is free to do so. What Catholic League President William Donohue is complaining about here is the fact that others are publicly celebrating Christmas in a manner which he disapproves of. Atheists, for example, are able to partake in Christmas without bowing to Christ.
What this comes back to is questions about the "real meaning" of Christmas. Donohue alludes to this when he asks why people have to "tippy-toe around the religious meaning of Christmas every December." For many Christians like him, there is only one meaning to Christmas: the religious meaning. The idea that there could be a nonreligious meaning to Christmas is either completely foreign to them or an outright insult to their religion.
Taking offense at a nonreligious meaning to Christmas is not an argument against it existence, though. Christmas has become as much of a cultural holiday for America overall as it is a religious holiday for Christians specifically. This cannot be denied and it cannot be changed on the contrary, the nonreligious aspects of Christmas will only grow as America becomes increasingly secular and increasing pluralist in terms of religion.
A further problem with Donohue's position is the fact that the government doesn't have the authority to single out his religion and his religious celebrations of his religious holiday for special endorsement or promotion. It doesn't matter how many Christians there are in America or how many Christians celebrate Christmas in a religious manner having a majority who support a religious practice doesn't magically give the government the authority to endorse or finance that practice.
When Donohue says that "97 percent of Americans say they are not offended by Christmas celebrations," he doesn't say if they were specifically asked about the government supporting and paying for religious symbols and celebrations which is, after all, precisely the sort of thing he argues for. Perhaps that 97% was expressing support for the generic, non-religious Christmas we see around us all the time.
Because of this, Donohue should arguably be happy that there are such extensive secular aspects to Christmas which the government can endorse and promote. Because those secular aspects overlap to a great extent with Christians' religious celebrations, this means that the government is indirectly supporting Christian observances of Christmas (like, for example, making Christmas a federal holiday). If it weren't for the largely secular nature of Christmas today, almost all of that support would be withdrawn and Christians would be entirely on their own.
History teaches that when Christians in America don't have the support of the government in their activities, it's much more difficult for them to maintain what they are doing. Christians recognize this, too, at least at some level. This is one reason why some work so hard to obtain or retain government financing and support for what they are doing. That's effectively what Donohue is doing here: demanding greater government and cultural support for his religion.
It won't work, though, because people are happy with the generic, nonreligious Christmas of modern America. That's why most Christians themselves also partake in it. When was the last time you saw a Christian reject Santa Claus, Christmas trees, and other non-Christian aspects of the holiday?