It’s the vernal equinox today at 12:57 PM EST, when the center of the Sun is in the plane of the Earth’s equator. So, why aren’t the lengths of day and night equal? Look up the times of sunrise and sunset today and you’ll see that the time between sunrise and sunset is longer than twelve hours, which is due to two things.
First, the convention for specifying the time of sunrise holds that it is when a bit of the Sun first appears above the horizon. Likewise, the time of sunset is when the last bit of the Sun disappears below the horizon. These make the day longer by the time it takes the whole (half rising and half setting) Sun to cross the horizon.
Second, the light from the Sun is refracted by the Earth’s atmosphere, making the Sun appear above the horizon when it is actually just below, in the same way that the stopper at the bottom of a sink filled with water looks to be higher (the sink looking shallower) than it is. The displacement of the Sun’s image near the horizon is slightly larger than its angular size, making the day longer by twice (rising and setting) the time it takes the Sun to cross the horizon.