Short answer

• Take pachchakkhan (pratyākhyan) for the “kāraṇ” (rules/conditions) before you begin that specific act, and never after sunrise if it affects food or drink.

Basic rule

• Pachchakkhan is a vow you declare in advance. It must be taken:

- in presence of a Guru; or, if not available, facing the image of Arihant or in samayik at home, - with clear time limits and conditions, - before starting the activity it governs.

Common timings (Shwetambar and Digambar both follow this principle; wording/ritual may differ)

• Upvās/Poshad: take the vrat pachchakkhan before sunrise on the day of the fast. If you receive it from a Guru, you may take it the previous evening; do not delay beyond sunrise.
• Āyambil “kāraṇ” (all the specific rules of āyambil): take the kāraṇ-pachchakkhan just before beginning any preparation/eating for the āyambil meal. If someone else cooks for you, take it before they begin cooking for you.
• Ekāsan/Beāsan: take the meal-specific kāraṇ pachchakkhan before the meal begins.
• Bhojan/Paṇī (food/water) daily vows: take before the first intake of the day; for strict observances (e.g., chauvihār), take it before sunrise.
• Vihār (travel): take just before starting the journey.
• Davai (medicine): take when needed, before consuming the medicine, stating dose and limits.
• Any special niyam (e.g., no night-eating, no green vegetables, etc.): take in advance for a set period (day/fortnight/month) and renew on its expiry.

Notes on traditions

• Shwetambar: householders commonly recite set pachchakkhan sutras; timing remains as above—always prior to the act, and food/drink-related before sunrise.
• Digambar: the pratyākhyan for āhār/ras-tyāg is likewise taken before sunrise and before the act; formulations may differ, but the timing principle is the same.

If unsure, keep the safe practice: take the relevant kāraṇ-pachchakkhan first thing in the morning (before sunrise) and again immediately before the specific act (e.g., before cooking/eating the āyambil meal).