Short answer (Shwetambar view):

• In Shwetambar Jain tradition, the main Moksha (nirvana) festival is Mahavira’s Moksha at Pavapuri, observed as Diwali. Gautam Swami, Mahavira’s chief disciple, is said to have attained keval-jnana (omniscience) very soon after Mahavira’s Nirvana; this keval-jnana event is often linked with the Diwali period in many communities, and is sometimes celebrated the day after Mahavira’s Nirvana as part of the same festival cycle. This is the general pattern you’ll find in Shwetambar celebrations around Diwali, with Gautam Swami’s attainment noted as a key spiritual milestone on that multi-day observance. There can be regional variations in exact rituals and dates. ( msjs.org.au)

Important notes:

• The core Moksha reference in most Shwetambar calendars is Mahavira’s Nirvana; Gautam Swami’s keval-jnana is treated as a closely related, celebratory event that occurs around the same festival period.
• Digambar and Shwetambar traditions differ in some details of festivals and the sequencing of events; the above reflects common Shwetambar practice as seen in temple calendars and community celebrations. For a Digambar perspective, you would often see different emphasis or dating for Gautam Swami’s attainment.

You can read more about how Jain communities connect the Diwali festival with Mahavira’s Moksha and Gautam Swami’s keval-jnana here