Just a quick idea [HAH! "quick" he says!
]:
It is possible that the depressed mood of people only associates with 'the holidays' because that is the time of year when it occurs [around the Winter solstice].
But the source of it may be seasonal affective disorder, which may affect a larger percentage of the population than is generally believed. [Exposure to broad-spectrum sunlight-equivalent lamps would assist with that.]
However, it is also possible that after years of people thinking that they were depressed thanks to 'the holidays', through associated learning they have come to believe that it is the holidays that are the source for their feeling down so strongly that they (and media) - effectively - have caused themselves to feel that way with the holidays, even if that isn't the original source for the feelings.
People who are 'into' the holidays generally have adrenaline bursts to prevent themselves from feeling the 'down' feeling from seasonal affective disorder, but when the willpower-push through all the shopping and crap ends, their overused adrenal glands collapse and they collapse with them.
[Don't have the energy to make the words "seasonal affective disorder" and "associated learning" link to WikiPedia entries for them.]
[Now the Psychologist will fade back into the woodwork, as I'm still operating crisis-to-crisis here, and can't afford the energy to write in detail.]
Always had doubts about S.A.D. because I wonder if it's really about it being cold, causing most people to stay inside...which results in boredom/depression, etc. In other words, is it because of the amount of sunlight, or is it because you can't lounge around the pool and drink beer? (Cabin Fever. "Snowmen have peepers! Peepers to watch!")
I'm a day sleeper, so I hardly get any sun, regardless of the season. I still have more anxiety/depression during the holidays than during other seasons. So, I'm certain, in my case, it's not the amount of sunlight that is the issue.
When I would get the holiday blues, my mind would reflect upon my life, and I would wonder about what could have been- had I gotten married, had I not been injured in the military, what if I had kids, etc.
I attribute that to media and societal pressure, because that line of thinking always ends on January 1st. Halloween, Turkey Day Christmas and New Year's Eve are "family and society oriented" holidays, and remind me that I am alone. Alone meaning, no wife or kids.
On January 1st, the depression would immediately lift. So, again, in my case, I doubt it was the amount of sunlight. It has to be societal/media and my reactions to them.