Though some people dread having to see their family around the winter holidays and don't enjoy Christmas shopping and the colder weather, others feel more depressed after the holidays are over, since they have to go back to the same routine, among other haunting realities. This is known as post-holiday depression. This can start after Christmas or similar holidays and/or after New Year's Day, depending on when a person celebrates.
According to an article on www.psychcentral.com, the possible causes of post-holiday depression are "unmet expectations, unrealistic resolutions, and a return of loneliness and guilt about overindulgence." For example, perhaps someone trys to give all her friends and family members gifts
The author of the article suggested that people treat the holidays no differently than any other time of the year. Just because it's Christmas, Hanukkah or another holiday, it doesn't mean everyone has to pretend to be happy when they're not. That can be depressing by itself.
Post-holiday depression generally only lasts for a short period of time. If the depression lasts longer, it can be classified as a more "serious" type of depression, though all types of depression should be regarded with care. According to an article on Suite 101, post-holiday depression is considered more of a mental distress than a mental disorder, since it's so fleeting and makes a minimal impact for the most part on the person who has it.
The idea that post-holiday depression is short-lived can be found in an article from www.PennLive.com. The article compares seasonal affective disorder to post-holiday depression, saying that "SAD isn’t the post-holiday blues people tend to get in January or the cabin fever resulting
from being indoors for too long on shorter days and in inclement weather."