Bhīṣma Pañcaka 2186

Bhīṣma Pañcaka is a five-day vrata observed from Kārtika Śukla Ekādaśī through Pūrṇimā, named for Bhīṣma Pitāmaha, who is said to have taught the Viṣṇu Sahasranāma and other discourses to the Pāṇḍavas during these very five tithis while lying on his bed of arrows, before departing on Uttarāyaṇa Pūrṇimā.

Observers typically take a vow of restraint for the duration, worshiping Viṣṇu each day and often eating only foods appropriate to a simple, purificatory diet. Because it spans a fixed set of tithis rather than a fixed set of calendar dates, the exact start and end day shifts slightly with tithi vṛddhi/kṣaya each year; this page resolves that for your city automatically.