I think if an item is geared towards a specific holiday (like red and green for x-mas, or something for Thanksgiving, etc) then putting "Christmas Gift" in your tags makes sense and is totally allowed.
If you item isn't holiday specific, then I kind of feel like it shouldn't be tagged for a holiday. From a shopper's perspective, say you typed in "Christmas ___" to search...how would you feel if something not at all related to x-mas popped up in your search results.
You could still tag your items with things like "Black Friday Etsy" and "Cyber Monday Etsy" if you're going to do a special for either of those shopping days, which will help you be found when people are looking for a holiday gift.