Fasting is an ordinance of the Church, obliging the Christian to observe it on specific days... ( Saint Nektarios of Egina )

Fasting is an ordinance of the Church, obliging the Christian to observe it on specific days. Concerning fasting, our Savior teaches: "When thou fastest, anoint thine head, and wash thy face; that thou appear not unto men to fast, but unto thy Father Who is in secret: and thy Father, Who seethe in secret, shall reward thee openly." 

From what the Savior teaches we learn 

(a) that fasting is pleasing to God, and 

(b) that he who fasts for the uplifting of his mind and heart towards God shall be rewarded by God, Who is a most liberal bestower of Divine gifts, for his devotion.

In the New Testament fasting is recommended as a means of preparing the mind and the heart for divine worship, for long prayer, for rising from the earthly, and for spiritualization.

Saint Nektarios of Egina