Devibhagavatapurana 3.27.13

3.27.13

saptamyāṃ ca tathā'ṣṭamyāṃ navamyāṃ bhaktibhāvataḥ |
trirātrakaraṇātsarvaṃ phalaṃ bhavati pūjanāt ||

On the seventh, eighth, and ninth days, in these three tithis (lunar days) if one worships with devotion, one will acquire all the merits.