Answer

I couldn’t verify a single fixed “Siddhitap start date” for all of 2026. Siddhitap is described as a Jain austerity fast, not a fixed calendar festival: one Jain source describes it as a 44-day monsoon fast, and a 2026 Jain calendar lists Paryushana beginning on September 8, 2026, while also noting that Jain festival dates can vary by location. ( jainsite.com)

Explanation

So, if you mean the usual Siddhitap observance during the Paryushan season, the relevant 2026 period begins with Paryushana on September 8, 2026. But I could not confirm one universal Siddhitap start date from the sources I checked. ( drikpanchang.com)

Takeaway

Siddhitap is a sacred tap, and its exact start is not listed as one fixed all-Jain date in the 2026 calendar sources I found. ( jainsite.com)