Why won't my door close in the winter? Please, take this question seriously, because this is a real problem to me. I have a door in my flat. A closet door, to be specific. And there is a problem with it. 
In the summer everything is ok, the standard wood door opens and closes as predicted, but in the winter time it drives me crazy! It just won't close! Seems that it doesn't fit where it belongs. Damn door just doesn't allow me to welcome guests and take a cup of mulled wine with them.
So the question is, what physical processes are standing behind that unfitting? I believe that sun and gravity may be involved, but I don't really understand it. So, please, help. If I'll know the problem, I'll figure out a solution.
P.S Maybe it is important that I live in Kazan', Russia
 A: My two cents (sorry, I know I'm late to the party):
Humidity is not nearly as season-dependent as temperature (it changes much more erratically) so I think that these changes, which are definitely season-dependent, must depend on temperature. But how, considering that the door doesn't fit in the winter? (What we would expect is for the door to shrink in the winter, yet seemingly it gets bigger.) The problem, I think, is that the door frame shrinks in the winter more than the door does (most likely it has a higher thermal expansion coefficient). So both the door and door frame shrink slightly during the winter, but the door frame slightly more, resulting in a door that won't fit!
Any thoughts on this guess?
A: It's actually just that in winters or around that time water doesn't evaporate that fast. So, the cellulose in the wood expands due to the added water content and swells until it doesn't fit the door frame. (1)
It's really interesting just how beautifully everything plays out in nature. If you think about it that property probably saved a lot of trees from dying. If wood expands in moisture that means the tree will be taller in just the right conditions. It contracts that means the tree will be shorter in just dry conditions. That, probably, reduces the surface area of evaporation and helps the tree to survive. I might be wrong, but it's mind boggling just how awesome nature is. She rocks. :D
(1) Cellulose is a polymer and I guess the expansion might have something to do with hydrogen bonding interacting with it's structure.
A: I would say it has to do with humidity, since it is a wooden door. It probably gets too humid during the winter and the wood expands. If it had to do with temperature, it would be the opposite effect (it would expand during the summer when it is hot).
A: I have the exact same issue with my bathroom doors. It is generally cooler in there during the winter. Since its my bathroom, the doors suffer a heavy dose of water (its humid). But this problem ONLY occurs in the cooler months. Could it be the metal door hinges, handles, locks, nuts and bolts that contract?  If those mechanisms were to shrink in size, it would displace the door's positioning....thus preventing it from shutting.
A: I have a similar problem with a garage door (backdoor, not the car entrance). Here in California winters can be vert wet, while summers are bone dry. I've spent hours shaving wood off the edges, to keep it from getting too bad.
  Stresses and strains on other parts of the structure might also be to blame.
A: I have the same problem with my doors in Concord California.  The reason seems to be the cememt slab.  During the Dry weather the ground is dry and the house slumps.  In the winter when the soil is wet the house rises up and everything fits differently.  I could water my slab (plant close to the wall some water loving plants) or live with it.  Water costs and hassle keep me from doing that so it keeps settling.  You could plane off the door so it's a little smaller to make it fit in the winter and be a little small in summer.  Just observe where it sticks and with your wood plane shave a little off till you like it.
A: I have doors like that too.  Probably everyone in a climate where it gets reasonably cool does too.  Other interesting effects include cracks that open/close based on temperature.  Appears that the house shifts slightly during seasonal changes.  Possibly due to temperature differences that the house generates in the ground resulting in asymmetrical movement?  Humidity appears to be a near non-issue (it does have some effect, but never enough to make the difference between working and not working).
A: I have the same problem (in England), but it's entirely attributable to how wet the weather is. Very wet spells can happen in any season here. The wood in the door swells as it becomes saturated with water. A few days of rain won't do it, but a couple of weeks will.
