Short answer: Paryushan was not “started” on a particular calendar date by any one person. It is an ancient Jain observance rooted in the monsoon stay (Chaturmās/Varsha‑vāsa) prescribed by the Tirthankars and carried forward by early Āchāryas. From very early times of Lord Mahāvīr and earlier Tirthankars, monks stayed in one place during the rains and the laity intensified sādhana; from this context the annual Paryushan developed.

Key points by tradition

• Shvetāmbara: Paryushan is observed for 8 days and culminates in Samvatsarī (the great annual Pratikraman and universal forgiveness). Kalpa Sūtra pravachan during these days is traditional. There is no single human “founder”; it is a continuation of the Avasyaka and Chaturmās discipline preserved in the Āgamas.
• Digambara: The 10‑day Das‑Lakshanā Parva begins right after the Shvetāmbara Samvatsarī. It focuses on the ten supreme virtues. This, too, is not attributed to a single founder; it arises from the same ancient monastic‑lay discipline of the rainy‑season retreat.

Essence

• “Paryushan” means to “abide/come nearer” to the Self—intensifying śravaṇ, dhyān, tapas, pratikraman, and kṣamā. Its authority is traditional Jain scripture and ācārya‑paramparā, not a specific historical initiator.