Here’s the Jain view in simple terms:

• There is no universal Jain festival or event named “attam tapasya” set for December 13–15, 2025. In Jainism, “attam” (also aṭṭam/aatam) is a unit of time used in karmic and fast-penance discussions, not a fixed community-celebration. ( jainknowledge.com)

• If someone chooses to do a special fast called Attham (three days of fasting), it is an individual austerity you can undertake as a part of Siddhi Tap or related practices. Typically, this involves fasting for three days, having one meal (parṇa) in between, then another three days of fasting. This is described as a powerful, rigorous form of tapas to purify the soul. ( jainknowledge.com)

• In practice, “Attam” is used in discussions about how long beings in certain realms live or how fasting can affect karmic bondage. It is a theoretical/time unit rather than a fixed festival date. ( jainknowledge.com)

• If you are planning personal observance on December 13–15, 2025, you could consider an Attham-style fast (three days, with one meal in between), following guidance from a trusted Jain monk or senior Shrāvak, and then perform the usual pratikraman and Paushadh practices as per your tradition. Note that Digambar and Śvetāṃbar traditions may have nuanced differences in vrata and practice; consult your local temple or guru for exact procedures in your tradition. ( jainknowledge.com)

You can read more about Siddhi Tap and Attham-related practices here:

• Siddhi tap meaning
• One Attam can reduce how many years of hell