With the arrival of the season of Spring, it's a good time to remind ourselves why we have seasons in the first place. But before we answer that question, let's examine an even more important question. What makes a year? It sounds like a stupid question, doesn't it? Now, most people who live here on Earth, let's call them Earthlings for the fun of it, know that a year is 365 days. Well, the reason an Earth year is 365 days is because that's how long it takes the Earth to make one complete revolution around the Sun. Actually, it takes a little more than 365 days. Six hours more, to be exact. Therefore, every four years we're 1 day behind (6 hrs X 4 years = 24 hours) on our calendars, so that's why every four years we add 1 day to the year. That extra day is Feb. 29th, also known as Leap Day. So every fourth year we have a "Leap Year" which makes up for that extra 6 hours it takes for the Earth to make 1 complete revolution around the Sun. If we didn't add the extra day every four years, then after a period of 40 years, we'd be celebrating the 4th of July on June 24th, and Christmas would occur on Dec. 15th! So we really need that Leap Day every 4 years to make up for that extra 6 hours of revolution time, otherwise we'd have serious problems with our calendars.
Let's say you are looking down on our solar system, with the Sun in the center, and the Earth revolving around the Sun. In this frame of reference, the Earth revolves counterclockwise around the Sun. As the Earth revolves around the sun, the season changes every 3 months. And every year right around March 20th, the season of Spring begins. Three months later, as the Earth has made about a 1/4 of a revolution around the Sun, the season of Summer begins (around June 21st), and so on and so forth for the beginning of Autumn (around September 22nd) and the beginning of Winter (around December 21st). Every time the Earth makes a 1/4 of a revolution around the Sun, we change seasons.
The final question we have to answer is "What makes Winter cold and Summer hot?" The answer is the fact that Earth is tilted on its axis. As the Earth makes its journey around the Sun, it remains "tilted" off its axis about 23.5° to the right of center. It is this tilt which causes the Sun's rays to shine more directly on the Northern Hemisphere in the Summer, and less directly in the Winter. In other words, during the summer season, the Earth is tilted TOWARD the sun, while during the Winter season, the Earth is tilted AWAY FROM the Sun. So the sun's ray are more (less) direct on the Northern Hemisphere in the Summer (Winter). Additionally, this tilt also controls the amount of daylight we see during a typical 24-hour day. In the Winter, when the Earth is tilted away from the Sun, the daylight time is very short, whereas in the Summer, when the Earth is tilted toward the sun, the daylight time is very long.
So Winter and Summer are pretty easy to understand. During Winter the Earth tilts away from the Sun, so the sun's rays are less direct and the days are shorter. Whereas during Summer, the Earth tilts toward the Sun, so the Sun's rays are more direct and the days are longer. But what about Spring and Autumn. Well, during those seasons, the Earth is neither tilted toward nor away from the Sun. That's why those seasons have less extreme temperatures.