“Porsoo upvas” simply means “fast the day after tomorrow.”

• In common Jain usage, “porsoo/parso” = the day after tomorrow.
• “Upvas” = fasting (Upavāsa) — staying near the soul by renouncing food (and traditionally water) from sunrise to the next sunrise, with right intention and pachchakkhan (vow).

How it is done (simple Jain maryada):

• Take pachchakkhan for upvas on that day.
• No food the whole day; in strict upvas, no water too (sunrise-to-sunrise).
• Spend the day in samayik, pratikraman, svādhyāy, and kayotsarg; keep non‑violence and humility in thought, word, and deed.
• Do proper pārṇā the next morning as per tradition.

Tradition note:

• Both Śvetāmbara and Digambara lay practice recognize upvas as a core austerity (tapa). Details like water allowance and timings can vary by local maryada; follow your sect’s guidance and capacity.

For a short, clear explanation of Upvas and its rules:

• Upvas basics
• Is water allowed in Upvas?
• Simple Upvas pachchakkhan (Śvetāmbara)