Although it is easy to imagine the seasons neatly fitting into four equal lengths, according to NPR this isn't quite the case, they are all slightly different — and their duration changes depending on which hemisphere you are in.
Due to the elliptical orbit of Earth around the sun, at certain points in the year the planet is moving faster and shortening the season. However, the distance from our star has less impact on Earth's seasons than the planet's tilt, which means that summers are warm in the Northern Hemisphere despite being further from the sun. As it is moving slower, the spring-summer season is also actually longer, by about seven days.