Saying "HAPPY HOLIDAYS" goes back to at least the 1860's in this country. SEE INTERESTING LINK:
In recent years there has been talk out there with some people saying there is a "war on Christmas" and other such commentary. It has come to the point where some are literally insulted if you say "Happy Holidays." First, this saying is not a replacement for "Merry Christmas." Many, including myself, say both. "Holidays" comes from "Holy Days." "Happy Holidays" and "Season's Greetings" most likely come from the combined ideas that Christmas is in fact a season spanning over many days and that the New Year's holiday is also included in the meanings of these sayings. There is more than one holiday, so it's plural, and it's a season celebrated over a number of days, really from Thanksgiving to New Year's Day. In old England, kids away at boarding schools would say they were "going home for the Christmas holidays," plural, because it was literally several days. And of course we have all heard of the 12 days of Christmas. More recently, the idea certainly has expanded among many people to include the fact that we have other celebrations at this time of year too including Hanukkah, Kwanza, Solstice, sometimes Ramadan, etc. America is a diverse nation that has always worked to become more inclusive of all its citizens. That doesn't mean anyone is against Christmas or saying "Merry Christmas."
We have been saying "Happy Holidays" in this country since at least the days of the Civil War in the 1860's and probably long before that. "Season's Greetings" also goes back at least many decades.
Please see this link for some very interesting original-source pieces of American history with "Happy Holidays" right there in black and white. "Happy Holidays" is a very long American tradition, and it is rather baffling that anyone could be offended by it.
http://www.hburgjeremy.com/2011/12/history-of-happy-holidays.html
My wish is that we will all move past this manufactured controversy. There are bigger fish to fry.
Merry Christmas and Happy Holidays to one and all in this joyous season of light, love, and life. 
= new reply since forum marked as read
...swings back and forth in this country, going from inclusive civic piety (which is what I remember from my early childhood in the 1950s) that makes a point of not saying "Jesus" when "God" will do, to a really in-your-face "Jesus is the only one and you're all damned to Hell if you don't believe exactly as I do" thing, which is what we've been dealing with for the past 20 years. The latter is very defensive and not at all conducive to civic tranquility.
I was a salesclerk throughout my college years, mid-60s onward, and "Happy Holidays" was absolutely the norm. "Merry Christmas" was okay too, but Happy Holidays included everything through New Year, and also didn't leave out any Buddhists or Jews or other non-Christians the store owners might want to sell shiny stuff to.
I can only hope it swings back, and soon.
Happiness from Solstice to Solstice to everyone, and to all a good night! 
