According to the Vaiṣṇava calendar, there are many fasts, such as Ekādaśī and the appearance and disappearance days of God and His devotees. All of these are meant to decrease the fat within the body so that one will not sleep more than desired and will not become inactive and lazy.
SB 4.28.3536, Purport

The injunction to fast on Ekādaśī is especially meant for devotees; on Ekādaśī there are no restrictions regarding food that may be offered to the Lord.
CC Adi 10.71, Translation

One should observe fasting on Ekādaśī day (this occurs on the eleventh day after the full moon and the eleventh day after the new moon. On such days no grains, cereals or beans are eaten; simply vegetables and milk are moderately taken, and chanting Hare Kṛṣṇa and reading scriptures are increased.).
Teachings of Lord Caitanya

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