Chandrayana Vrata is an important vrata lasting for a month which commences on Shukla Paksha Pratipada or Padyami (First day of the bright half of a month) by observing fast for the whole day and breaking it in the night with one morsel of food. On the next day, i.e. ‘dwitiya’ (Shukla Paksha Dwitiya) one morsel of food is increased which means that instead of one morsel of food now two morsels are taken.
This way one morsel of food is increased on each of the following days till the fourteenth day (Shukla Paksha Chaturdashi) when the fast is broken by having fourteen morsels of food.
A total fast is observed on ‘Amavasya’ (dark moon). On the next day i.e. Krishna Paksha Pratipada (first day of the dark half of the month) one morsel is decreased which means that the devotee should break his fast with thirteen morsels of food.
This way one morsel is decreased on each successive day till ‘Chaturdashi’ when a devotee breaks his fast by having only one morsel of food. This is the way, how a Chandrayan vrata should be observed.