Demeter is the Greek goddess of the harvest, agriculture, fertility and sacred law.
She was one of the children of Cronus and Rhea, and sister to Zeus, Poseidon, Hades, Hera and Hestia. All save for Zeus were swallowed by their father, who later forced Cronus to vomid the five other gods out and united they overthrew him.
Later on she had a daughter, Persephone, was abducted by Hades. She was upset of this, and as goddess of agriculture and fertility, refused to fulfil her duties, which led to massive droughts and famines in the realm of the humans. Zeus, worried about humanity, contacted Hades and begged him to release Persephone. Hades agrees, but it is also revealed that Persephone had eaten a pomegranate, which confined her to the Underworld for half of the year.
The time of the year when Demeter and Persephone would be together became spring and summer, while the time when she returned to the Underworld would be autumn and winter.