Kartik Purnima (Karthik Purnimā) is the full moon day in the Kartik month. In Jainism its importance is more regional and varies by community, but some common points are:

• A day of reflection and voluntary austerities: many Jains fast or do extra prayers on this day as a form of spiritual discipline.
• Visiting temples and doing charitable acts: temples are often visited, and acts of charity, feeding the hungry, and aiding others are encouraged.
• Respect for Tirthankaras and Jain teachings: reciting prayers, performing puja, and listening to discourses about the Tirthankaras’ lives and virtues are common practices.
• No uniform universal significance: unlike some Hindu festivals, Kartik Purnima is not celebrated uniformly across all Jain communities. Different Jain traditions (Digambara and Śvētāmbara) may emphasize it differently or give it varying practical observances.

If you’d like, I can look for a JainKnowledge.com page specifically about Kartik Purnima and give you a precise reference.