Srimad-Bhagavatam: Canto 3 - Chapter 1 - Verse 11
Sanskrit:
तयो: समुच्चयो मास: पितृणां तदहर्निशम् ।द्वौ तावृतु: षडयनं दक्षिणं चोत्तरं दिवि ॥ ११ ॥
ITRANS:
tayoḥ samuccayo māsaḥpitṝṇāṁ tad ahar-niśamdvau tāv ṛtuḥ ṣaḍ ayanaṁdakṣiṇaṁ cottaraṁ divi
Translation:
The aggregate of two fortnights is one month, and that period is one complete day and night for the Pitā planets. Two of such months comprise one season, and six months comprise one complete movement of the sun from south to north.