In devotional service there are certain activities which are called determined, such as fasting on certain days, like the eleventh day of the moon, Ekadasi, and on the appearance day of the Lord. All these rules and regulations are offered by the great acaryas for those who are actually interested in getting admission into the association of the Supreme Personality of Godhead in the transcendental world. The mahatmas, great souls, strictly observe all these rules and regulations, and therefore they are sure to achieve the desired result.

Srila Prabhupada, Bg 9.14, purport.