Answer

If you mean the Siddhitap observed with Paryushan, the start date in 2026 would be September 8, 2026. The Jain festival calendar lists Paryushana Parvarambha on that date, and Siddhitap is commonly observed during Paryushan. ( drikpanchang.com)

Explanation

In the all-India Jain calendar listing for 2026, August 2026 shows Rohini Vrat and Agastya Arghya, while Paryushana begins in September, not August. Siddhitap is a Jain austerity often undertaken during Paryushan, so the relevant start is tied to that parva. ( drikpanchang.com)

Takeaway

For August 2026, there is no separate Jain-calendar Siddhitap start date in the source I found; the meaningful start point is September 8, 2026. ( drikpanchang.com)