Garudapurana 1.128.14

1.128.14

māṅgalyamabhiṣekaṃ ca malamāse vivarjayat /
darśāddarśasya cāndraḥ syāttriṃśāhobhistu sāvanaḥ //

Auspicious rites, and royal consecration: one should avoid [these] during an intercalary month.
From new moon to new moon is the lunar month; but by thirty solar days is the civil month;