In Jainism, upvas (fasting) is a significant spiritual practice observed by laypeople (shravaks/shravikas) and ascetics. The term "chovihar" refers to the practice of not consuming any food or water after sunset until sunrise the next day.

Regarding your question: It is not strictly necessary to do chovihar (avoid food and water after sunset) the day before an upvas. However, it is highly recommended and considered auspicious. The ideal practice is to observe chovihar the evening before upvas, as:

• It helps prepare the body and mind for the fasting day.
• It prevents unintentional harm to microorganisms that proliferate after sunset in food and water, aligning with the core Jain principle of ahimsa (non-violence).
• It is believed to enhance the spiritual benefits and discipline of the upvas.

In summary: While not an absolute requirement, doing chovihar before upvas is strongly encouraged in Jain tradition. Many Jains make it a point to follow chovihar before observing upvas for maximum spiritual benefit.