The cold weather that meant you had to change into a warmer coat, which then made you late. And who knows what this little delay in your schedule caused you to miss, maybe a bad bout of traffic, maybe an accident? Perhaps as the result of this you met a lady at the bus stop who you wouldn't usually see. Maybe she inspired you?
Granted, January is a pretty gloomy month (I was shocked to see a blue sky today, having forgotten what one looked like), but why is that particular day more depressing than any other? Well, of course, it's not. Blue Monday was actually invented by a psychologist to help promote some travel company.