Apara-pakṣa — The Dark Fortnight
Apara-pakṣa (also spelled Apara-paksha) is the second half of the Hindu lunar month, spanning the fifteen waning tithis from Kṛṣṇa Prathamā, the first tithi after Pūrṇimā, to Amāvāsyā (new moon). The term "apara" means "later" or "subsequent," marking this fortnight as the second of the two lunar phases. It is synonymously called Kṛṣṇa-pakṣa (the dark fortnight) because the moon wanes during this period.
In contrast to Pūrva-pakṣa (Śukla-pakṣa, the bright fortnight), Apara-pakṣa is traditionally considered less auspicious for beginning new ventures or performing Vedic ceremonies. However, it is not inauspicious for devotional observances. The Kṛṣṇa Ekādaśī — the eleventh tithi of Apara-pakṣa — is one of the two monthly Ekādaśī days observed by Śrī Vaiṣṇavas. The precise form of fasting, vigil, and worship follows one's health, eligibility, household practice, and Āchārya guidance; a complete fast and all-night vigil should not be stated as an invariant rule for every devotee.
The Amāvāsyā tithi that concludes Apara-pakṣa is the appointed time for pitṛ-tarpana (ancestral water libations) and śrāddha rites. The month of Āṭi's Amāvāsyā and the Mahālaya Amāvāsyā in Purattāsi are particularly significant in the annual observance cycle. The interplay of Pūrva-pakṣa and Apara-pakṣa structures the entire liturgical rhythm of Śrī Vaiṣṇava daily and monthly practice.