The Earth is not straight on its axle but tilted by about 23 ½ ° and it is this tilt that gives the Earth its seasons. During the summer months, the Northern Hemisphere is tilted toward the Sun and we have long, hot days. But six months later the Earth has travelled half way around the Sun and now the Northern Hemisphere is pointed away from the Sun giving us short, cold days. Of course, for the Southern Hemisphere the opposite is true, winter for them is June, July and August.
The shortest day of the year -the winter solstice- usually falls on 21 December. On this date the Northern Hemisphere is pointed to its most extreme away from the Sun and so the Sun appears to stay very close to the horizon throughout the day. At any point on or above the Arctic Circle the Sun will not rise at all. The longest day, or summer solstice, usually occurs on 21 June; on this day the Sun will appear at its highest point in the sky at midday and at any point on or above the Arctic Circle, the Sun will not set. The date of the solstices can alter by a day or two depending on whether it is a leap year and also where you happen to be on Earth when the solstice occurs.
Half way between the two solstices are two dates when there is exactly twelve hours of day and twelve hours of night. These days are known as the spring and autumn equinox. Equinox is Latin for equal night.
This year's spring equinox will occur on Monday 20 March at 11.29am. On this day the Sun will rise due east, it will be directly overhead at the equator and will set exactly due west, a perfect time to check your compass bearings.
The Sun doesn't rise at exactly 6am and set again at 6pm, this is because sunrise is when the Sun's disc first appears above the horizon and sunset is the last glimpse of the disc. But as it can take up to seven minutes for the whole of the Sun's disc to become visible the equinox day, is counted from the centre of the Sun, horizon to horizon. Also the Earth's atmosphere has a refracting effect which means the Sun is visible for several minutes before it has risen and after it has already set. But after this date, the days will gradually become longer than the nights and spring has arrived.