Short answer

• In the Shwetambar tradition, Mahavir Janma Vanchan is kept on the 5th day (Panchami) of Paryushan so that, in the midst of the eight-day tapas and pratikraman, the community collectively listens to the Kalpasutra’s Janma‑Kalyanak of Bhagwan Mahavir and renews right faith, vows, and inspiration for the remaining days.
• Digambar tradition does not keep “Janma Vanchan” during Paryushan; their 10‑day Das‑Lakshan focuses each day on one supreme virtue.

Why specifically the 5th day?

• Scriptural focus: The Kalpasutra is publicly recited during Paryushan. By tradition of our Acharyas, the Janma‑Kalyanak section of Bhagwan Mahavir is reached and read on Panchami. This centers the festival on the life and ideals of the Tirthankar.
• Spiritual purpose: Mid‑festival uplift. After a few days of pratikraman, svadhyay and fasting, the Janma Vanchan raises devotion (bhakti), strengthens samyak darshan, and motivates stronger anuvrats, upvas, and peaceful conduct for the remaining days.
• Community saiyam: The celebration is not a “birthday party”; it is a solemn remembrance meant to purify passions, emulate Mahavir’s virtues (ahimsa, aparigraha, sanyam), and deepen inner restraint.
• Practical note: In many sanghs it is on the 5th day; occasionally local tithi/pravachan schedules may shift the reading by a day, but the intent remains the same—placing Mahavir’s Janma‑Kalyanak at the heart of Paryushan’s austerities.

Sect-wise clarity

• Shwetambar: Kalpasutra paath throughout Paryushan; Janma Vanchan from the Kalpasutra on Panchami with pothi‑pujan and bhav‑puja.
• Digambar: Observe Das‑Lakshan; no Janma Vanchan during these days—focus is on the ten virtues and Kshamavani at the end.

For a simple reference on “which day” and Kalpasutra’s importance, see:

• Which day of Paryushan is Mahavir Janma Vanchan?
• Importance/meaning of Kalpasutra in Paryushan
• Paryushan: purpose and spirit