Vaishnava Dharmashastra 90.12

90.12

āṣāḍhyām āṣāḍhāyuktāyām annapānadānena tad evākṣayyam āpnoti ||

If on the full moon day of the month Âshâdha the moon is seen in conjunction with the lunar asterism Uttarâshâdhâ and he gives food and drink (to a Brâhmana) on that day, he renders (the satisfaction effected by) them imperishable.