PROCEDURE FOR FASTING

Voluntary fasting on certain days is called "upavasa". The derivative meaning of the word "upavasa" is to remain in  proximity to lishvara: that is to say, to keep one's mind absorbed in the thought of lishvara. Fasting compulsorily on all "ekadashii" days is a must.Sannyasiis and renunciates must also fast on all purnima  and amavasya days.
On a fast day no food is to be eaten from sunrise until sunrise of the next day. Drinking water is also forbidden on fast days. If, due  to unavoidable circumstances, a person is unable to fast on the prescribed day, then he or she must fast on either the preceding or the following day. During sickness, fasting is not required, but in such cases written 
permission will have to be obtained from the Dharma Pracara Secretary of  the Samgha.
On and around the times of new and full moon, one may observe that the gaseous and aqueous factors in the body rise up into the head and chest, creating an uncomfortable feeling. Therefore, if a person does not take food at these times, these factors will be drawn from the  higher portions of the body to the lower portions, thereby alleviating
the uncomfortable feeling.
The food that we take is converted, through transformation, into its final essence, called "shukra". Shukra is the food of the brain. From it, the ectoplasmic particles of the unit mind are produced. If one fasts according to the system, no excess shukra will excite the 
lower vrttis of the mind, and the mind will be led towards the higher vrttis. Furthermore, as a result of fasting, the poisonous and  unnecessary waste of the body gets destroyed and expelled. Moreover, the energy that is not expended in digesting food can be utilized for other purposed. Therefore, a fast day is an excellent time for sadhana.