If by "Attam tapasya" you mean the Jain practice called Attham (three-day fast) as used in Siddhi-tap or similar austerities:

• What it is: Attham tapasya refers to a short, intensive fast typically lasting three days, often undertaken in two segments with a single meal (parna) in between, as part of a broader festival of tapas (austerity). This is one form of tapas used for purification and spiritual progress. Different Jain traditions describe details slightly differently, but the core idea is disciplined fasting and self-control. This is discussed in Jain texts and Jain study resources, including explanations of Siddhi-tap and its structure. You can read a concise explanation here: .

• About the dates December 13–15, 2025: Dec 13, 14, and 15, 2025 are three consecutive Gregorian dates. In many Jain calendars, these dates could be observed as a three-day Attham tapasya if someone chooses to perform a three-day fast starting on December 13. However, the exact observance (start time, whether it’s a continuous three-day fast or two Attham periods with a meal in between) can vary by local tradition and personal health. For precise local practice and tithi (lunar day) alignment, consult your temple or community calendar.

• Traditions: There are slight differences between Digambara and Śvetāmbara interpretations in any given rite. The essence—tapas, purification, and self-restraint—remains central in both.

If you meant something else by "Attam tapasya" or want the exact tithi alignment for Dec 13–15, 2025 in a specific Jain calendar (Śvetāmbara or Digambara), tell me which tradition and I’ll outline the exact details in that tradition.