सायन-सङ्क्रमण-दिन-अपराह्ण-पुण्यकालः

When the transit of the Sun (saGkrAnti) happens in the second half of the day (aparAhNa) and before midnight, the second half of the day form a puNyakAla for various observances. In addition to 16 ghatikas on either side of the transition, and specific puNyakAlas like SaDazIti, viSNupadI etc., this is also useful, especially in cases where the other puNyakAlas are inconvenient for appropriate performance of rituals such as snAna, dAna, tarpaNa, etc.

While the nirayana puNyakAla-s are well known, marked by the transit of the Sun into various rAzI-s (which confer the name to the corresponding saura month), there are also ayana-s and sAyana saGkramaNa-s, which mark the position of the Sun above the Northern or Southern parts of the Earth. These determine the Rtu-s, and also the very famous uttarAyaNa and dakSinAyana (movement of the Sun towards North or South, respectively). While these two types of puNyakAla-s (sAyana and nirayana) coincided millennia ago, there has been a gradual precession of the equinoxes, owing to the nature of the Earth’s rotation, causing the sAyana saGkramaNa-s to happen about 24 days before the nirayana saGkramaNa-s. In that era, uttarAyaNa coincided with makara-saGkramaNa and dakSinAyana coincided with karkaTaka-saGkramaNa, and the mEsa and tulA saGkramaNa-s corresponded to the equinoxes, when the Sun rises direct East (and sets direct West).

Many Dharma Shastra texts recognise the separate puNyakAla-s for both types of saGkramaNa-s. The vachanam of Jaabaali Maharshi is mentioned above, where he says that the puNyakAla-s are identical (in their reckoning) for both types of saGkramaNa-s. In the Pulastya Siddhanta, the anushthanas which are to be performed in this sayana punya kala just like nirayana are explicitly stated: snAna, dAna, japa, zrAddha, vrata, hOma, etc. In Kala Madhava, which is considered as the source text of the kala vicharas of the Dikshitiya, it is said in the 5th prakarana that, meṣādi-saṅkrāntayo yasmin dine bhavanti tasmād dināt pūrvebhyaḥ ekādaśabhyo dinebhyaḥ prācīne dine, noting that Sayana Sankramanas occurred ~11 days before the day when the Mesha Sankramanas etc occur. Further references are seen in Dharma Sindhu first pariccheda, Jayasimha Kalpadruma sankranti nirnaya and Nirnaya Sindhu first pariccheda. There are also ample references in Vedas, illustrating how Ayana-s happened much later (in certain other eras), e.g. Shukla Yajur Veda Shatapatha Brahmana (Ekapaat Kanda 1/2/3), Nakshatra Sukta belonging to the Atharva Veda (Shaunaka Samhita 19/7/2), Maitrayaniya Aranyaka (6/14), and Taittiriya Brahmana (1/5/2).

For a wonderfully detailed exposition on the topic, see https://sites.google.com/view/nakshatradarsha/dakshinayana-en and https://sites.google.com/view/nakshatradarsha/uttarayana-en .