In Blighty, it's when a glimpse of sun peeks through the clouds, usually around early May. This, combined with a very slight upwards shift in temperature, causes people of all shapes, sizes and ages to shed their clothes turning the country into sea of wobbly white (soon to be bright pink) legs, bellies and arms.
If you're lucky enough to escape the metropolis, you can witness other tell-tale signs of Spring, such as over-60's driving 40mph down main roads with the roofs down on their soft-top cars, and farmers taking to their fields in gargantuan machinery which smears the countryside in animal shit.
Then a week later it fucks off and it's back to perpetual Autumn.