Ah, Tit Monday. It's not that far off now, that glorious day when, heading into work on the bus, or walking to the Tube, or sitting on the train, you find yourself suddenly chirpier than you have been in months. You find yourself smiling at strangers again. There is a mild, involuntary tumescence in your trousers that comes and goes throughout the morning with the comforting regularity of a heartbeat.

And then you get a text around lunchtime from a mate which says: "At last, Tit Monday!" And you instantly understand why you are so happy. For Tit Monday is that special day in the year when, for the first time, the temperature rises above that magical point which causes girls getting dressed in the morning to decide to show a bit of skin.

After months of dull colours and chunky knit, the world's birds suddenly dive into last summer's wardrobe (they've not had chance to buy this season's stuff) and chuck it on without a thought. Your urban landscape is suddenly lightened with acres of naked arm and leg and, after many dark months of burrowing, breasts rising to the surface like moles at dusk.

Big breasts in white work shirts straining at the buttons. Small breasts braless in vest tops, the nipples fronted by ribby fabrics. Breasts in summer dresses bouncing in the distance so that they catch your eye before you even notice there is someone wearing them. Breasts nudging out from the crowd at traffic islands, quivering to cross the road...

And you know it is nearly summer. For previous generations, the arrival of spring was heralded by the sound of the first cuckoo. For us, it is Tit Monday.

Not that it always falls on a Monday. Like Easter, Tit Monday is a moveable feast. Last year it fell on a Friday. Friday 29 April, to be precise, when temperatures maxed out at 22.1C after nothing much above 16C all year. It last fell on a Monday in 2004, when temperatures leapt to 22C on 24 April.

And then, of course, there is Tit Monday Night. You see, in early summer, temperatures drop off very dramatically when night falls (Tit Friday 2005 dropped away to a parky 11.8C). But the dollies are not prepared. Slightly stunned by the morning heat, they drag out the summer clothes but forget to bring a cardie (a mistake they will not make again until next year), so that when they're all standing outside All Bar One after work celebrating the arrival of spring, their barely covered nipples have no protection from the cold. It's like a Bring-and-Buy sale where everyone has brought hat pegs. It's like a prog-rock gig where, instead of lighters, everyone is holding up nipples.

So when will Tit Monday fall this year? Will you be the first to text your mates with the announcement? Do not shoot your bolt too early. There will be false starts. You will smell fresh cut grass and see a couple of early starters and feel compelled to declare Tit Monday. But your more level-headed friends will tell you to hold your horses, keep your powder dry, don't fire until you see the whites of their bra straps As the poet said: one bold Northern slapper in a bikini doth not a summer make.