quick answer:

• Fasting on Astami is one of many vrata (self-imposed disciplines) practiced by some Jains to purify karma, discipline the body, and cultivate greater awareness and compassion.
• The eighth day (Astami) is viewed as a suitable time to discipline desires, reflect on one’s actions, and earn punya (spiritual merit) through restraint.
• It is not universal for all Jains. Whether an individual or a family observes Astami fast depends on local tradition, sect (Digambar or Shwetambar), and personal devotion.
• Two main ideas behind such fasts:

- Samyam (self-control) to reduce karma accumulation. - Bhakti and remembrance of the Tirthankaras, especially during sacred periods when fasting is common.

If you’d like, I can share more about how Digambar and Shwetambar traditions explain Astami observances, or provide a simple outline for a beginner’s Astami vrat.