There are two basic reasons for fasting.
To develop willpower and spirituality.
This reason relates to health, such as
purification and detoxification of the body
reduction of fatigue and sleep disorders
attaining a state of inner balance and harmony
Long fasts should only be carried out under supervision. Spiritual fasting should be practiced under the guidance of the Master, and fasting for physical wellbeing must be with medical supervision.
Here we will primarily deal with the first type of fasting - fasting to aid one’s spiritual development. If one wishes to fast for reasons of health, or weight reduction, one may do this at any time according to the given medical recommendations. However, when fasting for spiritual development there are certain rules and regulations to observe. The preparation and structure of the fasting days and one’s mental attitude are of greater importance in the spiritual fast. Any action performed consciously has greater influence upon the mind and psyche than any activity performed unconsciously. For the spiritual aspirant, fasting is not just to forgo one or two meals, but above everything else means to overcome bad habits, negative qualities and thoughts, and to restrain one’s self physically and mentally from any unnecessary activity. The fasting day should be a conscious, selfless, physical and mental sacrifice. If all of mankind lived as Vegetarians and fasted one day each week, the hunger problems, as well as many health problems, in the world today would be resolved.
Fasting has no religious basis, rather it is a training for self-discipline and an opportunity to gain greater self-knowledge. We strengthen our inner willpower by fasting and through this, can better manage our life in the world. We come to know our deepest, innermost thoughts and also gain the ability to persevere with what we have set out to do. Developing the ability to complete a self-imposed task is especially important for Yoga aspirants so that they can carry out their daily practices regularly without inner “discussion”. By fasting we gain sensitivity and intuition. We raise ourselves above the gross, material plane and intensify our contact with the subtle elements and spheres. Through this we can better understand the vibration and effects of the Cosmic powers.
The days on which we should fast are determined according to the position of the planets and the phases of the moon. The planets exert a tremendous influence upon our lives, but the changing phases of the moon have an even greater impact on the flow of energy in the Cosmos. In the Cosmos, two strong powers affect the earth - the power of the Sun and that of the Moon. The Sun symbolises consciousness and the Moon symbolises feelings and emotions. Feelings are a thousand times stronger than consciousness. It is these energies that change and move the world.
The radiation of the sun is constant, therefore its effect is always of the same strength. The effect of the moon however, is a hundred times stronger, but also far more variable than the sun. The phases of the moon influence nature as well as consciousness.
The ocean is often used as a symbol for inner peace, because its depths are always motionless and peaceful. However, at the time of the full moon when its radiance is at its strongest, even the ocean is set into motion. The highest waves and strongest storms mostly occur one day before or one day after the full moon. The moon also makes itself felt in both animals and humans. Cats, dogs, wolves, tigers and many humans are especially restless on full moon nights. They are affected emotionally and behave like the waves of the ocean. Women are quite strongly influenced. On full moon days women may become more sensitive, a little emotional, sometimes depressed or nervous. The moon is symbolic of the feminine energy principle and that is why a woman’s natural disposition is intensified at the time of full moon. However, these expressions of emotion do not mean a loss of power, rather a more intensified flow of energy.
But why do many people fall into depression on the full moon?
Those who become depressed are unable to allow their feelings and energy to flow outward. Their energy is blocked, therefore they feel nervous, touchy and despondent. However, precisely on the day of the full moon there exists a great opportunity to inwardly purify. The strong energy that streams into us on these days can release energy blockages and one’s energy can be restored to its natural flow. This functions like a drain or a channel where much rubbish has accumulated and become blocked. In order to clean it, one takes a hose and flushes out the drain with a strong jet of water to sweep away all refuse. In just this way the energy of the moon can have the same effect on blocked feelings and complexes. When we strengthen this effect by repetition of Mantra and fasting we purify our inner functions (Antahkarana) - our mind, intellect, consciousness and ego.
Just like the night of the full moon, the night of the dark phase of the moon has its own specific influence. The directly opposite effect of the full moon occurs at this time. All inner movements and stirrings of emotions are repressed and inhibited. Charitable activities (Paramartha) can unblock and release these emotions and purify them. Paramartha means unselfish action. To sacrifice something for others. For example, one can give half of one's food to someone more needy, or a donation can be made to needy animals. In other words, one hands over to another something that belongs to oneself.
On the dark, moonless nights there are certain activities that one should not perform, such as pruning plants, or laying the foundation stone for a house. It is because these acts go against the law of nature. When one unwittingly violates this law, the action is generally a failure.
The cycle of the moon is as follows.
It begins with the day of the full moon however, this day itself is not counted. The 15 days following the full moon are known as the “dark nights”. The day after the full moon its influence has already weakened and continues to weaken up to the 11th day. During these last four days the influence of the moon switches to the opposite effect. On the 15th day after the full moon the moon becomes invisible in the sky - this is the “dark night of the moon”. Two days later it reappears, very thin, as the “new moon”. In reality it already reappears on the 1st day, but its crescent is so thin that we are unable to see it. It is first visible with the naked eye on the 2nd day and begins from there to grow slowly. These are the “moonlight nights”. From the 11th day the effects of the full moon begin to unfold and on the 15th day after the dark night of the moon, the moon is full and the cycle is complete. As well as the day of the full moon and the dark night of the moon, the 2nd and 11th days in the moon’s cycle are the most suitable days for spiritual fasting, as on these days the influence of the moon is changing.
Fasting helps to develop and strengthen willpower. If one really wants to achieve something, one can fast until that goal has been reached. Many people who would like to realise something specific take a vow that until they have reached their goal they will fast. Through sheer willpower their wish is eventually fulfilled. Of course, when not eating for a whole day one naturally becomes hungry. You would like to eat but control the yearning of the senses with your willpower. For as long as your will is not strong enough, one may eat some nuts, fruit or milk. However, fasting not only develops willpower but one also learns to do without. One learns to sacrifice. Therefore, on this day you should give your food to others who are more needy.
On a fasting day one feels physically and mentally light and relaxed. You can concentrate more easily, things become clearer and you can use and direct your energy more consciously. Fasting on the days on which the influence of the moon changes (the 2nd and 11th day) has an especially purifying effect physically and mentally due to the intensified flow of energy.
Practice of the Spiritual Fast
How should you begin, continue and end a day of spiritual fasting? Since you already know beforehand that you are going to fast on a particular day, you need not buy any extra food. Simply begin to adjust yourself inwardly in preparation for the fast. In the morning light a candle, do your morning meditation and afterwards a few Asanas. Read a chapter from a holy book, then begin your day’s work.
Throughout the whole fasting day, be mindful that you are fasting for your mental and spiritual development. Be mindful of the promise you have made to yourself not to eat on this day. Within, you will feel happy and your self-confidence will be stronger when you keep your resolution to fast. Always be conscious that this is a special day and try to treat other people in a particularly friendly and positive way. When you come home in the evening, wash again, change your clothes and begin to prepare Prasad. Traditionally this is a sweet dish or milk dish, such as milk rice (Keer), sweet semolina (Halva), or something similar. Besides this you should also cook the normal evening meal for yourself and your family. The Prasad, as well as the meal, is prepared with much love, positive thoughts and with the singing of Mantras or spiritual songs. When you have finished cooking, place flowers, fruit and the Prasad on your altar or meditation place. Light a candle and a stick of incense. Light is symbolic of the presence of the Supreme. It represents knowledge and wisdom. The flowers, fruit and incense are an offering to the good powers in the Universe.
Then sit before the altar, either alone or with family or like-minded friends. Read a spiritual text from the Bhagavad Gita or the Bible, sing a spiritual song and say a prayer. Then distribute Prasad  with the following Mantra:
OM BRAHMARPANAM BRAHMA HAVIR, BRAHMAGNAU BRAHMANA HUTAM
BRAHMAIVA TENA GANTAVYAM, BRAHMA KARMA SAMADHINA
The act of offering is Brahman (God).
The offering (the food) is Brahman.
He who makes the offering is Brahman.
The fire (of digestion) is also Brahman.
Brahman is reached by those who recognise Him in all actions.
(Bhagavad Gita IV/24)
After this you have your evening meal and so concludes the fasting day.
Spiritual aspirants and Yogis always live as vegetarians - this means they eat no meat, no fish and no eggs. They avoid alcohol and all types of drugs. If you are (still) unable to follow this rule, then you should at least try to do so on the fasting day, once a week or once a month on the full moon.
In principle it is recommended that one maintains a spiritual fast on a particular day each week. Monday is the day of the Moon and Lord Shiva. Thursday is the day of Jupiter and the spiritual Master. A fast should also be maintained on the full moon day of each month.
It is essential on the spiritual fasting day, not only to abstain from meals, but also to spend the whole day with a positive, spiritual attitude. Only this will ensure that the fasting day is a spiritual practice and promote spiritual development.
On the fasting day, avoid intake of solid food during the day and only have a light meal (milk rice, Halva, rice, vegetables) around 5 o'clock. If this is not possible due to constitutional or health reasons, for example if a strong feeling of weakness or headache occurs, you may take some milk, fruit or nuts.
Naturally, herbal tea or water can be taken any time as needed.
If your health does not permit you to fast for a whole day, for example with Diabetes, then simply carefully plan to reduce or skip one meal.