Answer

For this year (2026), Posh Dashmi Atham Tap is observed around Posh Vad 10 (Dashmi).

The exact Jain tithi date can vary by location, sunrise tithi, and Jain panchang tradition, so I should not give an exact calendar date unless verified from your local Jain panchang.

Explanation

Posh Dashmi Atham Tap generally means observing Atham Tap connected with Posh Dashmi, which is especially associated with devotion to Shri Parshwanath Bhagwan.

Atham Tap commonly involves fasting or austerity for three days, usually around the Dashmi tithi as guided by one’s guru, sangh, or panchang.

Takeaway

Please confirm the exact 2026 date with your local Jain temple / sangh panchang, because Jain tithi observance depends on the local sunrise-based tithi.