The first day of Spring is in 2015 on March 20th.  It is also called the spring equinox.  At the spring equinox or the first day of spring, days are close to 12 hours long with day length increasing as the season progresses.

In spring, the axis of the Earth is increasing its tilt toward the Sun and the length of daylight rapidly increases for the relevant hemisphere. The hemisphere begins to warm significantly causing new plant growth to "spring forth," giving the season its name. 
 
Note: The date March 20, 2015 is calculated for Eastern Standard Time (UTC-5). It can vary in other time zones