tayoh samuccayo masah
pitrnam tad ahar-nisam
dvau tav rtuh sad ayanam
daksinam cottaram divi
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The aggregate of two fortnights is one month, and that period is one complete day and night for the Pita planets. Two of such months comprise one season, and six months comprise one complete movement of the sun from south to north.