I was interested in seeing just how old the phrase is. Nothing specific in OED but Salon has a relevant article albeit on the concept rather than the exact phrase. It quotes from a 1959 pamphlet issued by the John Birch Society (there’s an icy blast from the past, you don’t hear much of them now, thank goodness) and speaks of ‘the assault on Christmas’. The villain, hilariously, is the UN which has hatched a nefarious plot to replace all the Christian iconography in homes and store windows, etc with UN symbols. (Really, you couldn’t make this stuff up.)
“The UN fanatics launched their assault on Christmas in 1958, but too late to get very far before the holy day was at hand,” the pamphlet explained. “They are already busy, however, at this very moment, on efforts to poison the 1959 Christmas season with their high-pressure propaganda. What they now want to put over on the American people is simply this: Department stores throughout the country are to utilize UN symbols and emblems as Christmas decorations.”
It can’t have been very long for the phrase ‘war on Christmas’ to become the standard. I’m trying to recall when I first heard it. Perhaps the Reagan/Thatcher years? That may well be a false memory though.
Neutral Christmas greetings are as old as the hills though, with the diplomatic compliments of the season having a cite of 1766 in OED.
1766 T. Wroughton in H. Ellis Orig. Lett. Eng. Hist. ii. IV. 507 24 Dec. I heartily wish you the Compliments of the Season.
Happy New Year to all of you!