If you mean Varshitap / Siddhitap-style tapasya, the traditional start in 2026 is Chaitra Shukla Ashtami. In 2026, that tithi falls on Thursday, 26 March 2026. Jain Knowledge states the start should be on Chaitra Shukla Ashtami, and a 2026 tithi calendar shows Chaitra Shukla Ashtami on 26 March 2026. ( jainknowledge.com)

If your local sangh follows a slightly different panchang, please follow that local Jain calendar. ( jainknowledge.com)