I think it’s easier to define what isn’t a holiday rather than what is a holiday
A day of celebration for a specific event witnessed by a group of individuals en mass?
Should be something easily recognizable to anyone though.
For example, today is National Cookie Day in United States of America (Dec 4th, 2017). I wouldn’t necessarily call this a holiday since not everyone knows about it, nor is it widely celebrated.
However, if you want to do a Halloween themed art piece, vs a Day of the Dead, well both would be considered a Holiday (people know about both, or have heard of them, and do celebrate them in a festive manner).
¯\_(ツ)_/¯ a bit open to enterpretation I suppose.