Varshitap (Varsi Tap) for 13 months is a very high-level Jain tapa. I’ll explain it simply and practically.

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1. What is Varshitap (in your case, 13 months)

• Varshitap = alternate‑day fasting for about 1 year (traditionally counted as 13 lunar months + 13 days – that’s why people say “13 months”).
• You fast one day, eat the next day, and continue this pattern till the end date fixed by your guru/tradition.
• It is inspired by Bhagwan Rishabhdev’s long fast, which ended on Akshay Tritiya with sugarcane juice.

You can read a clear explanation here:

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2. Basic pattern to follow for 13 months

Exact rules can vary slightly by Digambar / Shwetambar and by local guru‑parampara, but commonly:

(A) Fasting days (Upvas / Khamasamana)

• No food all day.
• Only boiled water at fixed times (often only in day-time, not at night).
• Spend more time in:

- Navkar Mantra - Samayik (equanimity) - Pratikraman (repentance) - Swadhyay (study) - Jaap of Rishabhdev / Adinath

(B) Eating days (Biyasana / Ekasana day)

• Either:

- Ekasana – one sitting, one meal, simple and limited - Or Biyasana – two meals (morning + noon), still simple and pure

• No eating outside home/derasar as far as possible.
• Very simple food, made with full care of ahimsa, cleanliness, and aparigraha (no show‑off, no luxury).

More details on method here:

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3. Spiritual rules (very important)

During full 13 months, you should try to keep:

• Ahimsa: extra care in walking, speaking, eating, work.
• Satya: no lies, no harsh speech.
• Brahmacharya (celibacy): during Varshitap, no sexual activity, even with spouse.
• No anger, ego, greed, deceit as far as possible – this is the real tapa.
• Less social media / entertainment; more bhakti, swadhyay, and silence.

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4. Start and end (tithis)

Typical Shwetambar tradition:

• Start: Chaitra Shukla Ashtami (8th bright day of Chaitra).
• End / Parna: next year on Vaishakh Shukla Tritiya – Akshay Tritiya, with sugarcane juice in memory of Rishabhdev’s parna.

But:

• Some sampraday start on slightly different tithis.
• Digambar practice and counting can also differ a bit.

So for the exact 13‑month schedule, always follow the Panchang + guidance of your own guru or Jain sadhu/sadhvi.

For date-related info:

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5. Health & guidance

Because you are planning full 13‑month Varshitap:

• Take permission and guidance from a Jain guru / learned person.
• Check health with a doctor who understands long fasting.
• Arrange family support, because daily work + strict Varshitap can be difficult without help.

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6. Simple inner intention (bhav)

Throughout 13 months, keep this feeling:

• “I am doing this not for name, fame, or show, but to:

- reduce my passions (kashaya), - shed karma, - walk a little closer towards moksha, - and follow the path shown by Tirthankars.”

If you want, you may keep a very simple daily niyam like:

• Minimum 108 Navkar Mantra daily
• Short Samayik (even 12–24 minutes)
• Daily pratikraman (at least alochana-pratikraman by heart if not full text)

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In summary: Varshitap for 13 months = alternate‑day fasting with strict discipline, celibacy, and spiritual focus, done continuously from the fixed start tithi till Akshay Tritiya (or as per your guru’s instruction). The main thing is bhav + discipline, not just skipping food.