Brahmanas
The visuvant (summer solstice) period is 21 days in Aitreya Br. and 7 days in Pancavimsa Br., the summer solstice being in the middle of the period.
The gavam ayana ritual in SB 4.6.2. is based on the motion of the sun.
In the Maitrayana Brahmana Upanishad (6.14), the year is said to be into two portions, with the part from Magha to half of Śraviṣṭha associated with Agni, and the part from Sārpa to half of Śraviṣṭha associated with Varuna and Saumya (the moon). Aiyar has argued that Agni suggests the warm half and similarly Varuna the cool half of the year, suggesting the summer solstice at the beginning of Maghā and thus implying the vernal equinox in Kṛttikā. This, according to Kak, would correspond to 1660 BCE.
The Shatapatha Brahmana mentions that the Krttikas (the Pleiades) "do not swerve from the east". This would have been the case with precision at 2950 BCE and was true also about 2000 BCE, but was still true to within 8-13 degrees (viz., East by north) around the 8th to 6th centuries BC, the assumed date of the text's composition.
Read more about this topic: Archaeoastronomy And Vedic Chronology