According to Jain tradition, Lord Mahavira was born on the 13th day of the bright half of the month of Chaitra (Chaitra Shukla Trayodashi). It is celebrated as Mahavir Janma Kalyanak. As per Jain scriptures, Lord Mahavira was born on a Sunday. However, it is important to note that the exact day of the week may vary depending on the calendar system and historical calculations, but many Jain texts mention Sunday as the day of his birth.