Traditionally seasons occur on solstice and equinox but the reality of the matter is that, while it is an astronomical cycle, it isn't a good way to measure the change in season. The earth is constantly radiating energy in the infrared spectrum. During the day, it receives more energy then it radiates and along with the fact that there is an atmosphere, it warms up. At night, the earth radiates more energy than it receives. As the seasons change, the earth receives different amounts of daylight resulting in a shift in the energy budget resulting in an annual change in climate known as seasons.
Based on this energy budget, we know that while June 21, for example, is summer solstice, the hottest days of the year occur a few weeks after. There is a lag between when the earth absorbs a maximum amount of energy and when it radiates a maximum amount. Weather patterns will also affect this. This concept can be related to lake effect snow, where the weather cools but the Great Lakes still have a little warmth to them in the fall and early winter but eventually "catch up" to the colder air temperatures.
With this in mind, since seasonal changes result in change in climate for a location, it is better to declare seasons at a point where the coldest weeks lie in the center of the winter and the warmest weeks in the center of the summer and not at the beginning of the season as we do now. Therefore today, March 1, should be the official day of spring and not March 21. This is arrived by prorating the weeks of the year with the center of the hottest, coldest, and mildest days, for the mid-latitudes where seasonal changes are most prevalent, lying in the middle of the seasons where they are supposed to be.
Therefore, while an astronomical cycle affects the energy budget by altering the amount of insolation in a location over the course of a year, the actual warmest and coldest parts of the year, based on climatology versus weather patterns, should be classified as the center of the summer and winter, respectively and the official start and end date for each season adjusted accordingly.
So while everyone else considers it still winter, it is Springtime in my book.