Mahavir Janm Vanchan is not fixed to a single Gregorian date. In Jainism, the birth of Mahavir (Mahavira Jayanti/Janma Kalyanak) is traditionally observed on the 13th day of the bright half of the Jain lunar month Chaitra (Chaitra Shuddha Triyodashi). Because the Jain lunar calendar shifts each year relative to the solar year, the corresponding Gregorian date can fall in March or April in most years.

Some people or communities, or certain regional calendars, note celebrations around September as well, but that is not the canonical birth celebration. September dates you may have seen are often associated with other important Jain observances (like Paryushan—which ends with Samvatsari, the forgiveness day) or with local community customs, not with Mahavir’s actual birth anniversary per the traditional Jain calendar.

Differences between Digambar and Śvetāmbar traditions can lead to variations in how the date is calculated or observed, but the core birth date in the sacred Jain calendar remains tied to Chaitra Shuddha Triyodashi.

If you’d like, I can look for a Jain Knowledge page that explains the exact calculation of Mahavir Jayanti in different traditions, and share a relevant link.