Christmas, hands down.
First of all, it's in winter, my favorite season (because of snow and the cold, which I function better in than the heat). Second, marketing bullshit aside (and the fact that people try to push this shit to start earlier than it's supposed to), Christmas is supposed to be a happy time, a season for giving and spending time with loved ones.
Also the smell of pine and the decorations + lights look awesome! At my past jobs, I always decorated my cubicle/office, lights included.