俱舍論記 15.117

15.117

「若爾何故至近住齋戒」者，難。若說律儀惡趣無有，何故經言龍受八戒？半月，謂十五日。八日，謂月八日。又解：半月，謂白半十五日及黑半十五日。八日，謂白半八日及黑半八日。又解：半月中八日，謂白半八日及黑半八日。

Regarding 'If so, why ... to ... the upavāsa precepts': this is an objection. If it is said that restraint does not exist in the miserable destinies, why do the sūtras state that nāgas take the eight precepts? 'Half-month' means the fifteenth day; 'eighth day' means the eighth day of the month. Another explanation: 'half-month' means the fifteenth day of the bright half and the fifteenth day of the dark half; 'eighth day' means the eighth day of the bright half and the eighth day of the dark half. Another explanation: the 'eighth day in a half-month' means the eighth day of the bright half and the eighth day of the dark half.