It is quite difficult to pinpoint a specific date at the last day of winter, but the Spring Equinox is commonly accepted as the day that winter turns into spring. This, along with the Autumn Equinox, is the day with equal amounts of day and night.
This is not to say that other days are unequal, but rather that other days of the year have slightly more sunlight than darkness, or vice cersa.
The Spring Equinox is called an equinox because of the word equal, so the amount of day is 12 hours, and the amount of night is the same, also 12 hours.
This typically happens on or around March 21 in the Northern Hemisphere and September 21 in the Southern Hemisphere.
For 2017, the Spring Equinox was predicted to occur on March 20 for the Northern Hemisphere and September 22 in the Southern Hemisphere.