First up - and something I've got to be as careful with as anyone.
Today is not "the most depressing day of the year". Depression is a mental illness which can range from mild to utterly debilitating. One day of the year being slightly fretful isn't even close. Not the same thing at all. Like claiming that stubbing your toe is leg-breaking. Claiming a grotty day is "depressing" trivialises a serious condition and upgrades being mildly miserable.
Also. It's not even that bad a day. Nights are shortening, and in a bit of sunlight this afternoon, with a bird singing and a Red Kite swooping overhead (probably after the songbird) it was almost spring like.
And then, look what we've got to look forward to! Orthodox Epiphany, and then Imbolc. It's like Christmas never ended. And the good news is, you don't have presents to buy or cards to send. John Lewis aren't running six-minute ads intended to make us better people. The sort of amateurs who throng the pubs in December aren't in them. And the smug gets who go "dry" for January have stopped going the pub, having a half of a tooth-eating, gut-rotting, fizzy drink and telling us how great it feels to be detoxed. No, by now they're sat at home, watching Celebrity Big Brother, sucking the contents out of leftover chocolate liqueurs and wishing it was February.
So cheer up, face the music and dance. It's a beautiful world, and even if it's not then it's the only one we've currently got. Don't let a stupid advertising campaign get you down. And don't confuse being s bit down with depression.