Posh Dashami is an important Jain festival connected with Parshvanath, the 23rd Tirthankar. Its main focus is three days of devotion, discipline, and reflection.

What makes it important

• It marks the birth-taknaanaka (birth/renovation) of Parshvanath and is observed as Parshvanath-jayantī on Poṣha‑daśamī.
• Jains perform special vows and austerities for three days (often including limited food or ayambil-style fasting) to cultivate self-control and detachment.
• The rituals typically include worship of Parshvanath, listening to sermons about his life, and singing hymns. In many communities, a lamp with 108 wicks is lit during the puja.
• Specific ascetic gestures are practiced in some traditions, such as Ekasana (single meal a day) and recitation of Navakar Mantras, along with narrations of Parshvanath’s teachings.
• The festival is celebrated by both Digambara and Śvetāmbara Jains, though the exact calendar date may differ by region and sect.

In short, Posh Dashami is a time for disciplined worship, reflection on Parshvanath’s virtues, and practices that purify the mind and help progress toward moksha.

You can read more here and here