In the old cottage gardens, potatoes were traditionally planted on Good Friday. Why, when Easter is in a different place each year, and the weather is to variable? Why - because it was one of the few days the agricultural labourer could rely on having as a day off!!!
Another old way of deciding planting times was to say you could put the potatoes in when you could sit on the ground with your trousers down. Not a technique we'd recommend in the relatively public space of the allotments - although, curiously, not unrelated to the navigational techniques of the Polynesian islanders! However, it is clearly a way of testing soil temperature - much more important for spuds than the air temperature.
You can take a modern approach and buy a cheap soil thermometer. Plant and sow when the soil temperature two inches down (about 5cm) is 5degC or above. Particularly important when sowing parsnips, which hate a cold soil!
Spuds put in now will be a little later but you've still time to get a really good crop. 'Early' varieties aren't especially hardy, they're just quick to mature - so put them in now and you'll be eating the results before you know it.