In Jain dharma, “Atham tap” (also written Attham / Atham) is usually understood in two related ways:

1. As a tithi (day):

- Atham = the 8th lunar day (Ashtami) of each fortnight. - It is considered an auspicious day for: - Upvas (full fast) or Ayambil - Extra pratikraman, paath, and svadhyay - Many people casually say “aaj Atham tap chhe” when they mean they are fasting on the Atham tithi. ( jainknowledge.com)

1. As a specific fast (tapas):

- Attham tap = three continuous days of upvas (about 72 hours) with proper saiyam (self‑control) and dharma kriya. - Traditionally, this three‑day fast is planned so that the last day falls on Atham tithi. ( jainknowledge.com)

Small tradition differences

• Shvetambar:

- Often allow only boiled water in limited time (usually daylight) during such fasts, depending on local maryada.

• Digambar:

- Many emphasise more nirjal (no water) fasting for such taps, unless an ācārya / guru gives different guidance. ( jainknowledge.com)

But in both traditions, the main purpose is the same:

• To stop new karma (saṁvar)
• To shed old karma (nirjarā)
• By combining fasting with pratikraman, jap, dhyān, and good conduct.

For a simple, detailed explanation of Atham and how to observe it, you can read more here