Depression is depression, let's start there. I've heard/read many people think that seasonal depression is a "lesser form" of depression. That it isn't a valid reason for someone to "be depressed". If you are one of these people, you can either find any other article to read or continue here with an open mind in order to understand. There is no "lesser form" of depression. Any and all types of depression should be taken just as seriously.
What is seasonal depression?
Seasonal depression, also called seasonal affective disorder, begins and ends around the same time every single year. The most common time for seasonal depression to hit is from fall through winter. However, it can occur from spring through summer but it's rare. It has the exact same symptoms as "regular" depression: low energy, problems sleeping, changes in appetite or weight, difficulty concentrating, feeling hopeless, and even frequent thoughts of death or suicide.
What causes it?
Of course, there's no right answer to this. Causes vary from person to person. However, a reason may be your natural circadian rhythm. With the reduced level of sunlight in colder weather, it may disrupt your body's internal clock and result in feeling depressed. It may also be because of serotonin levels, also caused by lack of sunlight. Lastly, it can be because of melatonin levels. Understandably, if you already have depression, the changing seasons may make it worse as you are already predisposed to depression.
What about the holidays?
I can't speak for everyone, as everyone is different. However, personally, the holidays make my seasonal depression ten times worse. I was already in a bad place and having to either force myself to be happy for the sake of holidays/others or wallow in my depression and watch it make others uncomfortable is possibly the worst feeling in the world. No matter what path you choose, it still feels like the wrong decision so it's a lose-lose. It's honestly pretty common to feel this way, especially during the holidays. Now that the holidays are over, though, it's time for people with seasonal depression to try and get back up.
If you know someone with seasonal depression, you're going to have to be patient. Usually the personal with seasonal depression have no idea why they feel depressed, which is infuriating. Don't make it worse by trying to rush them or force them in a situation to fake happiness or invalidate them because "it's only seasonal." Believe me. It's hard enough to go through it without getting invalidated.