Answer

You can wish them:

“Jai Jinendra. May your Atthai tapasya be completed with shata, samadhi, and strong inner strength. May you receive the blessings of Arihant Bhagwan and progress spiritually.”

Other Simple Wishes

• “Jai Jinendra. Wishing you lots of shata and samadhi for your Atthai.”
• “May your Atthai be peaceful, spiritually uplifting, and completed with strong bhav.”
• “Michhami Dukkadam. May this tapasya help purify karmas and strengthen your soul.”
• “May Arihant Parmatma give you strength, peace, and samata throughout your Atthai.”

Meaningful Longer Message

“Jai Jinendra. As you begin your Atthai, may your mind remain peaceful, your faith remain strong, and your soul be filled with samata and devotion. May this tapasya bring karma nirjara, spiritual purity, and inner joy.”

Takeaway

The best wish for someone starting Atthai is to wish them shata, samadhi, samata, and spiritual strength, because Jain tapasya is not only physical fasting but also inner purification.