Punya (merit) in Jainism is earned by living the righteous path and performing virtuous acts. Observing tithis (lunar days) can be a favorable time to practice austerities and virtues, but the merit comes from the quality of the action and the intent, not from the day alone. Here’s a simple way to understand it:

• Core source of punya: Ahimsa (non-violence), satya (truthfulness), asteya (non-stealing), brahmacharya (celibacy or controlled conduct), and aparigraha (non-attachment). Acts that cultivate these qualities generate punya.
• How tithis help: Certain tithis are considered particularly auspicious for spiritual practice. During these days, performing virtuous deeds—such as fasting with restraint (upavasa), practicing regular meditation (dhyana), taking part in samayik (equal-minded reflection), sharing dāna (charity), and helping others—can amplify the effects of those deeds because the day is believed to support purifying karmas.
• What kinds of actions yield merit on tithis:

- Observing vrata (vows) or fasts with sincerity and self-control. - Engaging in bhakti or reverence toward the Tirthankaras with proper understanding. - Giving dāna to needy ones, especially through honest and mindful means. - Studying and reflecting on Jain dharma, and practicing right conduct in daily life.

• Important nuance: Punya is accumulated over countless lives and through consistent adherence to dharma. Observing a tithi can be a tool that helps focus effort, but merit is ultimately tied to the ongoing cultivation of non-violence, truth, restraint, and non-attachment.
• Tradition-specific notes: Digambara and Śvetāmbara traditions may emphasize slightly different practices or see different tithis as more or less auspicious for certain vows, but the fundamental idea—that virtuous conduct and disciplined practice generate merit—remains the same.

If you’d like, I can provide more precise explanations or sutra-level references from Jain scriptures (and note where Digambara and Śvetāmbara interpretations diverge) in a simple, easy-to-understand format.