Answer

The difference is mainly due to calendar tradition, not because the moksha event was different. Jain kalyāṇakas are tied to lunar dates, and Jain sources note that the recorded date can vary between Śvetāmbara and Digambara traditions, and even sometimes within the same tradition. ( jainpedia.org)

Explanation

In Jain calendrical practice, a kalyāṇak is identified by the month, fortnight, and tithi. Because a tithi is a lunar day and can shift during the civil day, different panchāṅg calculations may place the same sacred event on different Gregorian dates or even on different tithis, especially when one system uses the tithi at sunrise as the day’s tithi. ( jainpedia.org)

So for Bhagwan Neminath, both traditions honor the same moksha kalyāṇak, but they may preserve or calculate its commemorative tithi differently according to their accepted traditional sources. Jainpedia explicitly says such date-variation can occur between Śvetāmbara and Digambara sources. ( jainpedia.org)

Spiritual Understanding

In Jain Dharma, the most important point is the bhāva behind the observance: remembrance of Bhagwan’s liberation, devotion, and inspiration for liberation. The exact tithi is important in ritual practice, but the spiritual meaning remains the same in both traditions. ( jainpedia.org)

Takeaway

So the short answer is: the difference comes from sectarian calendrical tradition and panchāṅg calculation, not from the sanctity of the event itself. ( jainpedia.org)