Agni Purana 324.20

324.20

hrasvāḥ śuklā dīrghāḥ śyāmāṃstithayaḥpratipammukhāḥ /
udite śāntikādīni bhramite vaśyakādikam //

Short [vowels] are white, long ones are dark; the lunar days begin with pratipad; / when [the breath is] ascending, rites of pacification and the like [should be done]; when circulating, rites of subjugation and the like;