In Jainism, fasting (upvaas), especially during Paryushan, is considered a highly meritorious act and a means of self-purification and spiritual upliftment. However, Jain philosophy also emphasizes ahimsa (non-violence), including non-violence toward oneself. Caring for your body, especially during a period of healing or illness, is seen as important. Jain texts and Acharyas (spiritual teachers) often advise that one should not fast to the point of causing harm to the body or worsening a medical condition.

Regarding your situation:

1. Is it acceptable to fast during ACL recovery?

• Jainism allows for flexibility in religious practices when there are health concerns. If fasting could compromise your health or hinder your recovery, it is recommended to choose a less strenuous form of tapa (austerity).
• The Jain principle of “Deha-raksha dharma” (protecting the body is also a dharma) applies in your case.

2. Pros and cons of your current plan:

Pros:

• Fulfilling your religious intention during Paryushan.
• Engaging in spiritual purification and discipline.

Cons:

• Water fasting for multiple days, especially twice for 3 days each, may slow your healing process, reduce muscle strength, and hinder rehabilitation, as your body needs sufficient nutrition for recovery.
• Risk of low energy, dehydration, fatigue, or even potential setbacks in your physical therapy progress.

3. Possible modifications for greater success:

Jainism offers alternatives to food fasting for those with medical needs:

• Ayambil Tap: Eating one simple meal per day without spices, oil, or sweets, which is spiritually meritorious but less taxing physically.
• Ekasana/Biyasana: Eating once or twice a day, with simple, sattvic food.
• Non-food tapas: Engage in other forms of austerity, such as maun (silence), pratikraman (repentance and reflection), reading scriptures, meditation, or giving up favorite foods (ras-tyag).
• Intentionality: The spirit with which you undertake any vrata (vow) is important. Even a simple austerity performed with devotion is valuable.

4. What Jain scriptures and teachers recommend:

• Acharya Tulsi and contemporary Jain doctors have stated that one should consult with both a physician and a knowledgeable Jain monk if there is doubt.
• Jain scriptures do not consider it a failing if someone with a genuine health issue opts for a less strenuous austerity.

Summary

Your intention to fast is commendable, but your health and recovery from ACL surgery should take priority according to Jain principles. You can modify your practice by choosing a milder form of fasting or tapas that supports your recovery while still observing Paryushan in spirit and deed.

Consult your doctor and, if possible, a Jain spiritual guide (upadhyay or sadhu/sadhvi) for personalized advice.

Remember: _“Jeev daya karo, swasthya rakho, dharma karo.”_ (Show compassion to living beings, maintain your health, and practice dharma.)

Micchami Dukkadam! (I seek forgiveness for any unintended errors in this answer.)