Adinath (Rishabhanatha) is believed to have attained moksha on Kr̥ṣṇa Pakṣha, Kr̥ṣṇa Chaturdashi in the month of Magh (Magh Krishna Chaturdashi). In Jain tradition this day is observed as Nirvana (moksha) Kalyanak for Adinath, though the exact calendar can vary slightly between Digambar and Śvetāmbara communities. A common teaching is that moksha happened at Mount Ashtapada, with some regional celebrations focusing on Adinath’s Nirvana as a key kalyanak. You can read more here and related notes in Jain Knowledge discussions.