I first heard the phrase Happy Holidays as a child listening to Irving Berlin's song of the same title. He wrote the song in 1942, so the term has been around at least since then and as a 60 year old, I've been using the phrase since I was a kid. But I also said Merry Christmas and in my experience, way more people get sideways over me saying happy holidays than they do if I say merry Christmas. I almost always use the phrases together these days.