Śatapatha Brāhmaṇa ("of a hundred paths") 2.1.4.1

2.1.4.1

yadaharasya śvo 'gnyādheyaṃ syāt | divaivāśnīyānmano ha vai devā manuṣyasyājānanti te 'syaitacvo 'gnyādheyaṃ viduste 'sya viśve devā gṛhānāgacanti te 'sya gṛheṣūpavasanti sa upavasathaḥ

On the day preceding his Agnyâdheya, he (the sacrificer with his wife) should take his food in the day-time. For the gods know the minds of man: they are, therefore, aware that his Agnyâdheya is to take place on the morrow; and all the gods betake themselves to his house and stay (upa-vas) in his house; whence this day is called upavasatha (fast-day).