Well, to elaborate on what Stallzer said, it depends on whether you are selling "christmas stamps." I cannot imagine that stamps or other philatelic items are gifted sufficiently at Christmas time to make any noticeable impact on sales on
ebay. As for "christmas stamps," I'm not even sure there is really any kind of market for that on
ebay. Christmas stamps for postage use will be purchased at the post office. Interest among topical collectors of stamps and covers of past Christmas stamp issues might pick up at Christmas time, but unless that's your niche, you won't notice it.
So I would probably say, "It depends on what you are selling, and maybe not even then."