In Jainism, Ekasana refers to taking only one meal in a day, with certain rules about the timing and manner. Chauvihar means abstaining from both food and water after sunset until the next sunrise, while Tivihar allows water but no food after sunset.

Your question: Is it necessary to do Chauvihar or Tivihar the previous day to observe Ekasana the next day? If not, when should one stop drinking water on the previous day if planning for Ekasana, but without doing Chauvihar?

Jain Perspective:

• Chauvihar and Ekasana are independent vows/observances. It is not compulsory to do Chauvihar or Tivihar the previous day in order to perform Ekasana the next day. However, combining them is considered more meritorious and in line with stricter vows.
• If you wish to do Ekasana the next day without observing Chauvihar or Tivihar the previous evening, you can do so. There is no strict rule in Jain scriptures that mandates Chauvihar or Tivihar as a prerequisite for Ekasana.

About water intake:

• For a standard Ekasana, water can be taken before and after the meal, as per your usual daily limits, unless you are combining it with Chauvihar or Tivihar.
• If you wish to increase the austerity, you may voluntarily choose to limit or stop water intake after a certain time, but this is not a requirement for Ekasana alone.

Summary:

• You can perform Ekasana without having done Chauvihar or Tivihar the previous night.
• There is no strict rule in Jain Agamas regarding the exact time to stop water intake before Ekasana, unless you are also observing Chauvihar/Tivihar.
• Greater spiritual merit is believed to accrue when vows are combined (e.g., Chauvihar + Ekasana), but it is not compulsory.

If you have a specific sectarian tradition (Murtipujak, Sthanakvasi, Terapanthi, etc.), there might be minor variations in practice, so you may consult your local Jain monk or community elders for further guidance.