Complex sentence: The most important festival reserved solely for women was the Thesmophoria, a fertility rite for Demeter reserved for married noblewomen, during which women spent three days on Demeter's hilltop sanctuary performing rites and celebrating.
Split into simpler sentences: The most important festival reserved solely for women was the Thesmophoria, a fertility rite for Demeter reserved for married noblewomen. During this festival, women stayed for three days on Demeter's hilltop sanctuary, performing rites and celebrating.