I suppose the phrase "Happy Memorial Day" doesn't make a lot of sense, but it's hard to phrase it as a greeting otherwise. "Good Memorial Day," "Somber Memorial Day," "Reflective Memorial Day" are probably more accurate, but sound odd. Is there such thing as a "happy" Memorial Day if you're doing it right? I think so - in that there's a kind of peace in honoring sacrifice, in understanding that everyone who sacrificed something for you did it because they ultimately wished you happiness.
Anyway, Happy Memorial Day