Although it will feel quite spring-like Thursday and Friday, spring will officially arrive Sunday evening at 7:21 pm.
This is the vernal equinox -- when the direct rays of sun hit the equator, giving us equal day and equal night.
The perfect 12 hours of daylight and 12 hours of darkness occurs near the equator at the equinox. We actually experience that slightly earlier. Wednesday marks the last day with less than 12 hours of daylight. Thursday (St Patrick's Day) will have exactly 12 hours of daylight.
We will continue to gain 1 to 3 minutes of daylight each day until we hit the start of summer in June.