Short answer

• Take Pachkhan (Pratyakhyan) first thing in the morning during Paryushan, before you drink water or eat anything.
• For night‑time restraints like Chauvihār or Tivihār, take that Pachkhan before sunset.
• Choose the Pachkhan that matches your tapa for that day: Upvās/Ayambil/Ekāsan/Beāsan, Samayik, Paushadh, Chauvihār/Tivihār, etc.

A little detail (kept simple)

• What is Pachkhan: a clear vow you make for a fixed time, place, and rules. You state exactly what you will do and what you will avoid.
• When to take:

1) Day vows (Upvās, Ayambil, Ekāsan/Beāsan, “water only,” Navkārsi/Porsi limits): take in the morning, preferably before sunrise and definitely before the first sip of water/first meal. 2) Night restraint (Chauvihār/Tivihār): take before sunset. 3) Samayik/Paushadh: take just before starting the samayik/paushadh.

• Which Pachkhan to take (pick according to your capacity and your Sangh’s practice):

- Upvās (nirjal or sajjal), Attham/Atthai as applicable - Ayambil - Ekāsan or Beāsan - Chauvihār or Tivihār - Samayik - Paushadh (poshadh) - Food‑timing limits like Navkārsi/Porsi/Sād‑porsi, if you follow these

• How to take: in the presence of Guru; or in front of Jin Pratimā/Siddhachakra, standing or seated facing East/North, speak the resolve clearly (language of your tradition is fine), and then follow exactly what you vowed.

Tradition notes

• Shvetāmbara Paryushan (8 days): daily Pachkhan in the morning; Chauvihār/Tivihār before sunset; Samvatsari day has special Pratikraman in the evening.
• Digambara Das‑Lakshan (10 days): the same principle—Pachkhan in the morning for day austerities; night restraint before sunset. Names/wording may differ by Sangh, but the timing logic is the same.
• If your Guru/Sangh gives a specific text or time window (e.g., for boiled water), follow that as primary.

Helpful references from JainKnowledge

• When to take Pachkhan for sajala upvās
• Can Pachkhan be taken late in the morning?
• Upvās (water‑only) Pachkhan basics
• Paryushan timings (pratikraman, navkārsi, etc.)

Follow your local Acharya/Sangh for exact wording and sect‑specific nuances.