When something is Christmassy, it feels like Christmas — the look, the smell, the mood, not the calendar date itself. Red and green, lights on a tree, wrapping paper, pine, cinnamon, a jumper with reindeer on it: people call all of that Christmassy. A shop in November can already look Christmassy. It is a warm, informal word. Families say it at home; shops love it in ads. You would not put it in a serious report. Older and younger people both use it. There is no common opposite; you just say it does not feel like Christmas.