Galavasmriti 1.8: Vīramitrodaya · Nirṇayasindhu

1.8

tribhir vā divasair ūne dvābhyām ekena vā tathā | ādyādiṣu ca māseṣu kuryād ūnābdikādikam

Deficient by three days, by two, or by one, / in the first and subsequent months, one should perform the [rites] such as the ūnābdika.

When (a month) is short by three days, two or one, in the first and the following months, one should perform the 'deficient' yearly and the other (śrāddhas).