He wasn't drunk. He wasn't off to a party. There was no sense of "look at me I'm wearing a funny Santa hat". He was just reading his book as though the Santa hat was just the first thing he happened to have to hand to wear that morning.
Obviously there will be a fair few people in more outlandish Christmas outfits or festive headgear, but I liked the casual look of this guy. Below is one of my favourite pictures from a few years back.