These dates are mentioned at the end of this article.
Fasting On Purnima / Full Moon
Purnima, the full moon day, is considered auspicious in the Hindus Calendar and most devotees observe fast throughout the day and pray to the presiding deity Lord Vishnu. Only after a whole day of fasting, prayers and a dip in the river do they take light food at dusk.
It is ideal to fast or take light food on full moon and new moon days as it is said to reduce the acidic content in our system, slows down the metabolic rates, increases endurance. This restores the body and mind balance. Praying too helps in subduing the emotions and controls the outburst of temper.
Fasting on Amavasya / New Moon
The Hindu calendar follows the lunar month and Amavasya, the new moon night, falls at the beginning of new lunar month which lasts for about 30 days. Most Hindus observe a fast on that day and offer food to their ancestors.
According to Garuda Purana (Preta Khanda), Lord Vishnu is believed to have said that the ancestors come to their descendents on Amavasya to partake of their food and if nothing is offered to them they are displeased. So, Hindus prepare 'shraddha' (food) and await their ancestors.
Many festivals like Diwali are observed on this day too. Amavasya marks a new beginning. Devotees vow to accept the new with optimism as new moon ushers in the hope of a new dawn.
How to Observe a Purnima Vrat / Full Moon Fast
Usually, the Purnima fast lasts for 12 hours - from sunrise to sunset. People on fast do not consume rice, wheat, pulses, grains and salt during the duration this time. Some devotees take fruits and milk but some observe it rigidly and go even without water depending on their capability of endurance. They spend time praying to Lord Vishnu and conducting the sacred Shree Satya Narayana Vrata Puja. In the evening, after sighting the moon they partake of the 'prasad' or divine food along with some light food.
How to Perform a Mritunjaya Havan on Purnima
Hindus perform a 'yagna' or 'havan' on purnima called the Maha Mritunjaya havan. It is a significant and powerful ritual very simply undertaken. The devotee first takes a bath, cleanses his body and wears clean clothes. He then prepares a bowl of sweet rice and adds to it black sesame seeds, diced 'kush' grass, some vegetables and butter. Then he lays the 'havan kund' to strike the holy fire. On a designated area, a layer of sand is spread and then a tent-like structure of wooden logs is erected and smeared with 'ghee' or clarified butter. The devotee then takes three sips of the Gangajaal or holy water from the river Ganga while chanting "Om Vishnu" and lights the sacrificial fire by placing camphor on the wood. Lord Vishnu along with other Gods and Goddesses are invoked followed by the chanting of the Mritunjaya mantra in honor of Lord Shiva:
Om trayam bakkam, yajaa-mahe
Mrityor mooksheeya maamritaat.
The mantra is ended with "Om Swaahaa." While uttering "Om swaaha", a little helping of the sweet rice offering is placed on the fire. This is repeated 108 times. After completion of the 'havan' the devotee must ask for forgiveness for any mistakes he has unknowingly committed during the ritual. Finally, another 'maha mantra' is chanted 21 times:
Hare Krishna, Hare Krishna,
Krishna, Krishna Hare Hare,
Hare Rama, Hare Rama,
Rama Rama, Hare Hare.
At the end, just as the gods and the goddess were invoked at the onset of the havan, similarly after its completion they are requested to return to their abodes.