Question for the vets here: This is tagged as Christmas on Pixiv, but outside of the color scheme of the dress, this picture really doesn't scream Christmas (imo), so should it be tagged as such here? I seem to recall something about tagging only what you see, so...
That begets even more complication, because a lot of the time we draw tags from the artist (especially when we don't know something specific, such as a model of a gun). But, "tag only what you see" is part of the Danbooru creed.
Being a purist, I would not tag it as such. Someone who is more loyal to the artist's will probably would. That's how tag wars get started. :)
I myself have always been a "Artist is law" with tags. Fully knowing about Danbooru's "Tag what you see" rule. You do have to admit though, there are times when Artists should get the final say on things. For example, Traps/Reverse Traps and such.
The artist clearly drew this with the christmas theme in mind. The red and white colors and what looks like a christmas reef in the background. There really is no harm in adding the christmas tag especially since it says so on Pixiv.