Answer

Siddhi Tap does not have one fixed all-India start date in 2026, because it is an austerity/vow that devotees begin on their own sankalp date. In many Jain communities it is observed during Chaturmas/monsoon, and Paryushan in 2026 begins on September 8, 2026. ( jainsite.com)

Explanation

Jainsite describes Siddhi Tap as a progressive fast of one upvas, then two, then three, up to eight, with parna in between, and says it is a 44-day austerity with 36 fasting days. Since it is a tapas rather than a single fixed festival date, the start day can vary by person, temple, and sangh. ( jainsite.com)

Takeaway

If you mean the community Siddhi Tap observance connected with Paryushan in 2026, the nearest fixed reference is September 8, 2026. ( msjs.org.au)