Tomorrow is the spring equinox, which is when day and night are equally long. After tomorrow, the day will be longer than the night (in the northern hemisphere).
This is because of the way the Earth is tilted with respect to the Sun: As the Earth goes around the Sun, it ends up with the north pole either tilted towards or away from the Sun. On the summer solstice, the north pole is tilted as far towards the Sun as it can be, and so the northern hemisphere gets pretty direct sunlight and the longest day of the year, which warms things up and we have summer. At the winter solstice, the north pole is tilted as far away from the Sun it can be, so we get indirect sunlight and the shorted day of the year; things cool off - winter. Note that the opposite is true in the southern hemisphere: The south pole is tilted away from the Sun while the north pole is tilted towards, so southern hemisphere winter is during northern hemisphere summer.
In between the solstices we have the equinoxes, when the Earth is tilted neither toward nor away from the Sun. The spring (vernal) equinox is when the days are getting longer and the fall (autumnal) equinox is when the days are getting shorter.
Tomorrow, on the spring equinox, the Sun will rise (more or less) due east and set due west, and it will be up (more or less) the same amount of time as it's down. I say more or less because there's atmospheric lensing effects, but it doesn't make much difference.
The seasons are defined like this:
Spring - day is longer than night, and days are getting longer
Summer - day is longer than night, but days are getting shorter
Autumn - night is longer than day, and nights are getting longer
Winter - night is longer than day, but nights are getting shorter
So autumn and winter are the dark nighttime seasons, and spring and summer are the bright daytime seasons. Each season is 3 months long. Works for me.
Ah, and since tomorrow is the first day of spring, that makes today Winter Wrap Up Day! Have a video: