Some of us know of Pavlov while some of us don't. Ivan Pavlov (1849-1936) was a Russian physiologist who discovered, explored and exposed 'Classical Conditioning' to the world.
Classical Conditioning is a behavioral phenomenon, if an organism comes to associate one thing with another, for example associate a bell with food because the bell usually comes before food is served, then anytime the organism hears the bell even without food being served it's body would give out responses similar to the responses it gives when faced with food. Responses like salivation, excitement, a sudden urge/readiness to eat, an anticipation of food, etc.
Pavlov discovered this through his experimentation with dogs. He would always ring a bell before food is delivered to the dogs and with time the dogs were used to getting food soon after they heard the bell so their bodies started responding with salivation and anticipation of food, etc whenever they heard the bell.
This behaviour is also there in humans and one perfect example of how it works that I'll give is this: when you chose a particular song for your call ringtone and it stays that way for a long enough time then later you change it you'd find out that anytime you hear the song from your playlist or even when you're just passing by you would suddenly get the feeling that you have a call, sometimes you would find your self searching for your phone to pick up the call before you realize its not a call.
This is because overtime you've come to associate this song with your incoming calls that even after changing your ringtone hearing the song anywhere still elicits that response in you. Very interesting phenomenon isn't it?
Now, do you know that we have learnt to associate something with the Christmas holiday? That now once that thing comes around we start getting the holiday feeling? Truth is we have come to associate a lot of things with Christmas that they elicit that feeling from us whenever we experience them, but none of them is as strong as this one thing: weather!
Weather does change close to the Christmas holiday and the weather of December has come to create the Christmas holiday festive feeling in us. In most parts of the world it is winter. When people start feeling the winter air/cold they start to get the festive feeling. In Nigeria and West Africa it is the harmattan. The harmattan is a cold dry wind that moves from the Sahara through West Africa into the Gulf of Guinea usually from the end of November to March.
So that's it, whatever weather you have in December is usually one huge trigger for whatever feeling it is you usually get during the Christmas holiday, you start anticipating, getting in the mood and all.