The seasons reflect the Earth's tilt toward and away from the sun. These tilts are called solstices and equinoxes. They occur four times a year and, although their dates don't match exactly, they roughly correspond to the seasons.
Essentially, when a part of the Earth is pointed toward the sun, the weather is warmer and the days longer. When that part points away from the sun, the weather is cooler and the days shorter. The solstices occur in late June and December of each year, while the equinoxes take place in late September and March of each year. The positioning of the respective Northern and Southern Hemispheres during this times is what subsequently causes opposite seasons for them. The changes that occurred within the seasons were quite important to ancient civilizations because they helped farmers determine when it was time to plant fields and reap harvests. Until the Romans developed the calendar that is used widely today, constellations in the sky helped civilizations determine how close the Earth was to the respective solstices and equinoxes. Pegasus and Andromeda are most prevalent in the fall, Orion in winter, Leo and Virgo in the spring and Sagittarius and Hercules in the summer.