In Jain Dharma, Bhagwan Ṛishabhdev (Ādināth) is the first Tirthankar of the present time-cycle (avasarpiṇī).

About His birth:

• Place: Ayodhyā (called Vinītānagar in some texts)
• Parents: King Nābhirāj and Queen Marudevī
• Tithi (lunar date): Chaitra Krishna Navamī (9th day of the dark half of Chaitra)
• Kāl (time): According to Jain time‑scale, He was born an inconceivably long time ago, many sagaropams ago – so it cannot be converted into a normal historical year (like BCE/CE).

Because of this, Jains celebrate “Ṛishabhdev Janma Kalyāṇak” every year on Chaitra Krishna Navamī, not on a fixed English calendar date.

You can read a simple explanation here