As ordained by the deities human beings should take their food twice a day. viz., morning and evening. During the interval one should not eat anything. By following this rule about cating, one is said to observe a fast.
सायंप्रातर्मनुष्याणामशनं देवनिर्मितम्। नान्तरा भोजनं दृष्टमुपवासविधिर्हि सः॥