Digambaras and Śvetāmbaras celebrate Paryushan on different dates mainly because they follow different Jain calendars and calculation methods for the tithis (lunar days) and months.

• Calendar systems: Each sect uses its own traditional Panchang (calculation rules for lunar months, tithis, and auspicious days). Because the starting point of the month and how tithis line up with the solar year can differ, the same month in one calendar will shift to a different date in the other.
• Definition of the month: Some calendars begin the important period at a different tithi or lunar day, or place the start of Paryushan in a different month of the year, leading to a shift in the observed dates.
• Regional and historical variations: Over time, local traditions and historical decisions have solidified these differences, so the exact dates can vary by year and by country/region.

In short, the divergence is not about beliefs but about distinct, traditional ways of calculating and marking the Jain lunar calendar used by each sect. If you’d like, I can summarize the exact dates for a specific year and show how each calendar arrives at them.