The ChaAdrAyana vrata is one of the most difficult vratas to perform. Usually, the doer gets rid of almost all the possible sins he has committed. There are many difficult rules to be followed during this vrata hence very few people are actually take up this vrata. It is considered of great merit to perform this vrata.
During the krShNa-pakSha, the one undertaking this vrata or
penitential rite will decrease food consumed, by one mouthful each
day; similarly, with growing kalA of the chandra in shukla-pakSha, one
increases the quantity of food consumed. On the day of amAvAsyA, one
keeps a complete fast. Alternately, some smrtis talk of consuming 240
kabalas during the entire month.
Also, one is required to take a bath at every sandhyA (prAtaH,
madhyAhna and sAyam). This is the general description of the
chAndrAyaNa vrata. There are variations such as shishu chAndrAyaNa,
yati chAndrAyaNa etc for specific circumstances and adhikAra bheda.
The amount of food consumed is measured in terms of kabala or balls of
cooked rice, the size of which is required to be of the size of a
hen’s egg. The food consumed should be sanctified through gAyatrI
mahAmantra. Alternately, VaiShNava commentators on dharma shAstras
explain that the vrata starts with consumption of fourteen kabalas,
decreasing each day by one till ekAdashI, on which day one keeps a
full fast. On dvAdashI, fast is broken by consuming a single kabala,
gradually increasing the same by one every day during shukla pakSha.
While not always followed, traditionally it is said that one needs to
offer unhusked rice to a cow with suitable qualities and from the
undigested grains collected from the cow dung, cooked rice is prepared
for consumption during the vrata. Vrddha parAshara declares the vrata
as futile if such rice is not consumed during the vrata. ParAshara
also instructs one to begin the vrata from shukla pakSha and conclude
the same with brAhmaNa-santarpaNa. Based on whether the vrata begins
with increasing or decreasing chandrakalA, there are classifications
such as yavamadhya and pipilika chandrayana (Yajnavalkya).
Among the most auspicious times for fasting are the days of Full and
New Moon as well as the eighth and the twenty-third of each month
(counting from the first day after New Moon). New Moon Day (Amavasya)
is that day of the lunar month which begins when the last crescent of
the Moon becomes invisible at dawn - for which reason it may
accurately be called No Moon Day. The night before this, that is, New
Moon's Eve, is the Sacred Night of Shiva (Shiva Ratri) which is also
known as The Fourteenth Day of Shiva and The Dark Night of Shiva
(Shiva Kalaratri). Fasting during the day and night on this auspicious
occasion is particularly meritorious, and is observed for the
expiation of sins as well as for acquiring religious merit. (Source)