Saṅkrānti — The Sun's Monthly Transit
Saṅkrānti is the Sun's passage from one zodiac sign (rāśi) into the next. There are twelve saṅkrāntis in a solar year, and each begins a new solar month. Traditional pañcāṅgas identify an associated puṇya-kāla for the observances appropriate to that transit.
Makara Saṅkrānti and Uttarāyaṇa
The best-known is Makara Saṅkrānti, conventionally associated with the beginning of Uttarāyaṇa, the Sun's northward course. Bhagavad Gītā 8.24 speaks of the bright path in the context of the departure of qualified knowers and yogins. It is not a blanket rule that every person who happens to die during the calendrical Uttarāyaṇa automatically attains mokṣa. Śrī Vaiṣṇava teaching relates the liberated soul's passage to Bhagavān's grace and the archirādi path, not to an accidental date alone.
Dhanurmāsa in Śrī Vaiṣṇava Practice
During Dhanurmāsa, corresponding broadly to Tamil Mārgaḻi, Śrī Vaiṣṇava temples and homes give a distinctive place to Āṇḍāḷ's Tiruppāvai, often one pāsuram for each day of the month together with the applicable temple or household observances. Tiruvempāvai belongs to a different Śaiva devotional corpus and is not part of the Śrī Vaiṣṇava Dhanurmāsa recitation cycle.
Observances
Sacred bathing, Bhagavad-ārādhana, charity, tarpaṇa and other acts may be appointed for particular saṅkrāntis. Their exact timing, eligibility and procedure follow the pañcāṅga, śākhā-sūtra, family custom and Āchārya guidance. They are not one invariant set of obligations for every devotee.