On a practical level, all of the Sabbats take place at important time during the year for agrarian societies. All of the Sabbats give thanks for a harvest, or make an offering for a harvest to come. You may think that it doesn't apply anymore, we aren't all farmers, I got news for you, we are VERY reliant on farmers because we still do eat. Besides, a bountiful harvest for them, means lower market prices for us.
However, on the spiritual level, the sabbats represent different points in the Myth Cycle of the Goddess and God. Keep in mind, as you read about the sabbats, that the Goddess is considered to be transcendant (or operating outside of the terrestrial world) though when she chooses she can become immanent (operating within the terrestrial world) while the God is immanent and then becomes transcendent as He represents the cycle of life, death and rebirth.
Vernal Equinox/Ostara: March 21
Summer Solstice/Litha: June 21
Autumnal Equinox/Mabon: Sep. 21
Esabats are done 13 times a year on the full moon. Covens get together and party and cast spells. We give thanks for what we have recieved, and beseech the gods for what we wish to recieve. It is also a celebration where witches gather to share ideas about casting and upcoming Sabbats.
I really recommend that witches in covens practice this, it helps to solidify the covens union.
For witches that don't have a coven, this is a great time to caste for the lunar energies at their peak and spells have their greatest impact.