In 2026, Jain Chaumasa (Chaturmas) is starting on:

28 July 2026 – Ashadh Sud 14 (Chaumasi Chaudas)

Most Jain sanghs treat Ashadh Sud Chaudash / Chaumasi Chaudas as the formal start of Chaturmas for that year, so you can take 28‑07‑2026 as the Chaumasa start date.