Obligatory Prayer and Fasting

The fasting period, which lasts nineteen days starting as a rule from the second of March every year and ending on the twentieth of the same month, involves complete abstention from food and drink from sunrise till sunset. It is essentially a period of meditation and prayer, of spiritual recuperation, during which the believer must strive to make the necessary readjustments in his inner life, and to refresh and reinvigorate the spiritual forces latent in his soul. Its significance and purpose are, fundamentally spiritual in character. Fasting is symbolic, and a reminder of abstinence from selfish and carnal desires.  
(From a letter written on behalf of Shoghi Effendi to the National Spiritual Assembly of the United States, January 10, 1936; Lights of Guidance)