I've always thought of "holiday" as a day of celebration, which is something I don't attribute to Memorial Day or Veterans Day. Honestly, I didn't know there was more to the word "Holiday" than that so that's something I'm probably going to rethink.
Well, the word origin is "holy day." It's definitely taken on a more party-like connotation recently, but if you look at, say, some of the traditional holidays on a religious calendar like Hallowmass or Yom Kippur you'll see that it could be either celebration or quiet remembrance. So in that sense Veterans and Memorial Day fit.