Bharatiya Jyotihshastra (History of Indian Astronomy), translated by R. V. Vaidya v1.c5.59

v1.c5.59

After these two units, the day and the month, were firmly established, man must have observed that the rains, winter and summer recur after some definite period of time. It was also observed that the rains and other seasons recur after twelve months, where a ‘month’ denotes the period indicated by the full-moons. This (long) period of twelve months appears to have been called in the Vedas—Śarad, Hemanta etc. after the seasons in the Ṛk Saṃhitā. The term Śarad in the sense of a year, occurs there more than twenty times and the term Hima more than ten times. These words are found in many places in other parts of the Rigveda as well. The very word Varṣa meaning ‘a year’ also signifies a particular season.