Tithi — The Lunar Day and Its Theological Significance
Tithi ('that which crosses, that which is crossed'; from tṛ = 'to cross, to traverse') is the lunar day — the primary unit of time in the Vedic lunar calendar, and the first of the five aṅgas (limbs) of the Pañcāṅgam (five-limbed almanac). A tithi is defined as the time taken for the moon to advance 12 degrees ahead of the sun — since the sun and moon are moving at different speeds, each tithi has a variable duration (ranging from approximately 19 to 26 solar hours), meaning a tithi may begin and end at any time of day or night, and may occasionally span three solar days or be entirely skipped.
The thirty tithis of the lunar month are divided into:
- Śukla pakṣa ('bright fortnight'): the 15 tithis from new moon to full moon — Pratipad (1st), Dvitīyā (2nd), Tṛtīyā (3rd), Caturthī (4th), Pañcamī (5th), Ṣaṣṭhī (6th), Saptamī (7th), Aṣṭamī (8th), Navamī (9th), Daśamī (10th), Ekādaśī (11th), Dvādaśī (12th), Trayodaśī (13th), Caturdaśī (14th), Pūrṇimā (full moon, 15th)
- Kṛṣṇa pakṣa ('dark fortnight'): the 15 tithis from full moon to new moon, ending in Amāvāsya (new moon, 30th)
For Śrī Vaiṣṇava religious life, certain tithis carry special theological and liturgical importance:
Ekādaśī (11th tithi) is the most sacred tithi in Vaiṣṇava practice — observed as a day of fasting (upavāsa) and heightened devotion. The Bhāgavata Purāṇa, the Padma Purāṇa, and the Skanda Purāṇa all contain extensive māhātmya (glorification) literature on Ekādaśī. The theological understanding is that on Ekādaśī, the moon's influence withdraws the mind from external sense-objects — creating a natural condition of mental withdrawal (pratyāhāra) favorable for meditation on Bhagavān. Fasting on Ekādaśī is one of the prescribed observances for all Vaiṣṇavas, and the Śrī Vaiṣṇava tradition prescribes nirjala Ekādaśī ('waterless Ekādaśī') on Jyeṣṭha śukla Ekādaśī — the most severe observance.
Dvādaśī (12th tithi) follows Ekādaśī and is the day of Pāraṇa — the ritual breaking of the Ekādaśī fast. Pāraṇa must be performed within specific time windows on Dvādaśī morning. The Dvādaśī tithi is also marked by special arcana (worship) and the recitation of the Dvādaśa-nāma (twelve names of Bhagavān).
Amāvāsya (new moon) is dedicated to pitṛ-tarpana — the offering of water to ancestors — and to special śrāddha (ancestral memorial) rites. It is also the tithi on which Kūrattāzhvān composed his Śrī Stava and other stotras, and certain Āzhvār pāsurams speak of seeking Bhagavān's grace on this day.
Pūrṇimā (full moon) is auspicious for many Vaiṣṇava observances. The Śrī Vaiṣṇava calendar marks the full moon of Mārgaśīrṣa as especially sacred — it is the month Bhagavān declares as His own vibhūti in Bhagavad-gītā 10.35 (māsānāṃ mārgaśīrṣo'ham). The full moon is also the day for completing Satyavrata and Sātyakī observances.