The saṃhitā does not provide explicit formulation with regard to the equinoxes and solstices, although there are scanty references to the seasons. The details are given in Vedāṅga-jyotiṣa which comes in rc and yajur recensions, although according to Pingree this work could be quite late (c 400 BC). You shall find discussion on this topic in the following works:
Kuppanna Sastry, T S, K V Sarma, and Lagadha. 1985. Vedāṅga Jyotiṣa of Lagadha in Its Rk and Yajus Recension. New Delhi: Indian National Science Academy.
Pingree, David. 1981. Jyotiḥśāstra : Astral and Mathematical Literature. Wiesbaden: Harrassowitz. 8-10.
Subbarayappa, B V. 2008. The Tradition of Astronomy in India : Jyotiḥśāstra. New Delhi : Coronet Books Inc.