四分律比丘含注戒本 1.27

1.27

The Buddha said: 'One should follow the king, whether it is the fourteenth day or the fifteenth day. One should calculate the reckoning: the dark and the white [fortnights] each have fifteen [days]. Name them according to the month. If four persons are obtained, this motion should be performed; for three persons or fewer, the precepts should be recited with three declarations.'