Garudapurana 2.27.30

2.27.30

pretatvaṃ susthiraṃ tena mama jātaṃ nṛpottama /
ekādaśaṃ tripakṣaṃ ca ṣāṇmāsikamathābdikam /
pratimāsyāni cānyāni evaṃ śrāddhāni ṣoḍaśa //

Because of that, the state of a preta has become firmly fixed for me, O best of kings: / the eleventh-day rite, the rite of the third fortnight, the six-monthly, and the annual, / and the other monthly rites—thus are the sixteen śrāddhas.