SB 5.22.5
atha sa esa atma lokanam dyav-aprthivyor antarena nabho-valayasya kalacakra-gato dvadasa masan bhunkte rasi-samjñan samvatsaravayavan masah paksa-dvayam diva naktam ceti sapadarksa-dvayam upadisanti yavata sastham amsam bhuñjita sa vai rtur ity upadisyate samvatsaravayavah.
Translation by His Divine Grace A. C. Bhaktivedanta Swami Srila Prabhupada:
The sun-god, who is Narayana, or Visnu, the soul of all the worlds, is situated in outer space between the upper and lower portions of the universe. Passing through twelve months on the wheel of time, the sun comes in touch with twelve different signs of the zodiac and assumes twelve different names according to those signs. The aggregate of those twelve months is called a samvatsara, or an entire year. According to lunar calculations, two fortnights — one of the waxing moon and the other of the waning — form one month. That same period is one day and night for the planet Pitrloka. According to stellar calculations, a month equals two and one quarter constellations. When the sun travels for two months, a season passes, and therefore the seasonal changes are considered parts of the body of the year.