The 2016 astronomical spring for the Northern Hemisphere begins on Sunday, March 20. It will end on another Sunday in June.
This equal illumination of Earth by the Sun is the equinox,this time vernal and later this year autumnal.
You won’t fall over, but the tilt of the Earth’s axis is now inclined neither away from nor towards the Sun. The Sun is vertically above a point on the Equator.
Those ancients, who were better about paying attention to what was happening in the sky and on the ground, made the word equinox from the Latin aequus (equal) and nox (night), because the night and day are approximately equal in length.
You have probably been noticing the day lengthening already. I keep track of the Sun’s movement around my house. I have a chart and record the angle using a compass from a spot at my window. All winter, the morning Sun would come through the patio doors and shine on my chair in the family room where I sit to read and write. Now, it makes its early morning appearance in the east at another window and sends a beam on the couch where my wife has her morning coffee and reads the news.
In the Abrahamic tradition, the Jewish Passover usually falls on the first full moon after the northern hemisphere vernal equinox, although occasionally (currently three times every 19 years) it will occur on the second full moon. Christian churches calculate Easter as the first Sunday after the first full moon on or after the March equinox. The official church definition for the equinox is March 21. The Eastern Orthodox Churches use the older Julian calendar, while the western churches use the Gregorian calendar, and the western full moons currently fall four, five or 34 days before the eastern ones. The result is that the two Easters generally fall on different days but they sometimes coincide. The earliest possible Easter date in any year is March 22 on each calendar. The latest possible Easter date in any year is April 25.
I like knowing about other calendars, like the traditional East Asian ones, that divide a year into 24 solar terms. There the vernal equinox and the autumnal equinox mark the middle of the spring and autumn seasons, respectively.
In June the season will change again, not with an equinox, but with the summer solstice.