Fasting, which is sometimes done as a prescribed activity for specific religious observances such as festivals and vows (vrats), and is sometimes done as a way of expiating one's sins, is referred to as a general phrase.
Although upavasa may refer to complete abstention from food
and drink, it is most often associated with dietary changes.
In other situations, such as during the Shivaratri festival,
such "fasting" means refraining from cooked grains, which are
regarded such an important component of a meal that the term "rice"
is also used to describe "food" in some regions of India.
In other circumstances, one will refrain from eating
particular foods, such as the Santoshi Ma Vrat, which requires a person to
abstain from eating anything with a sour or bitter taste.
When fasting is done as an act of penance (prayashchitta),
the prescriptions generally involve the quantity of food consumed rather than
the kind.
The chandrayana, a penitential rite lasting one lunar month
in which the penitent's food consumption mirrors the monthly course of the
moon, is the most well-known of these.
On the first day of the declining moon, the performer eats
fourteen mouthfuls of food, then one fewer mouthful on each subsequent day,
culminating in a full fast on the new moon day.
The penitent eats one more mouthful each day during the
waxing moon, until he reaches fifteen mouthfuls on the full moon day.
