My children only wanted to hibernate by January. We've found leaving a challenging time as they (unsurprisingly!) resisted being frozen on the back of the bike. With the arrival of spring and the warmer weather they are more and more keen to leave the house. As well as the warmer weather, the addition of a car to our lives has also helped change leaving the house from a mission of many layers of clothes to a much easier transition!
The other system is astronomical seasons, based on the Earth's position in the universe compared to the moon and the sun. Both systems have their appeal and starting spring earlier is great news in my book. But then the end of summer comes earlier in the meteorological system....
Here is the article I read if you want to read more: http://www.metoffice.gov.uk/learning/learn-about-the-weather/how-weather-works/when-does-spring-start
For me the verges of spring flowers blooming and trees blossoming is the best sign of spring after a season lacking much new life.