CC Ādi 15.10
sei haite ekādaśī karite lāgilā
Mother Śacī said, “You have spoken very nicely. I shall not eat grains on Ekādaśī.” From that day, she began to observe fasting on Ekādaśī.
It is a prejudice among smārta-brāhmaṇas that a widow must observe fasting on Ekādaśī but a woman who is sa-dhava — who has her husband — should not. It appears that before Lord Caitanya’s request, Śacīmātā, being sa-dhava, was not observing Ekādaśī. Śrī Caitanya Mahāprabhu, however, introduced the system that a woman, even if not a widow, must observe the Ekādaśī day and must not touch any kind of grains, even those offered to the Deity of Viṣṇu.