Temperature, length of day, clocks changing, the return of migrating birds, there are many ways of defining the start of spring. For me it starts when it's warm enough to get plants out of their winter storage areas and into the garden again. The top comes off the cold frame, more cold sensitive plants get put out in the day to start preparing them for full sun, and the pot holders get filled again.
Given that winter pretty much puts my gardening on hold, spring holds a special place as once again I can do more than just looking at plants in the shed. For once my OH agrees, although for her it is because she gets her windowsills back and no longer has to worry about getting spiked as she walks around the house.