In Jainism, a 1-day Upvas (eka-upvas) is a short fast observed to purify and discipline the mind and body.

• General idea: abstain from eating solid food for one day, keeping the focus on self-control, penance, and inner reflection.
• Common practice (variations exist): you usually stop eating from a chosen time (often after sunrise) and resume the next day after completing the day’s fasting. Some people observe it from sunrise to sunrise; others may end the fast after a single, simple meal the following day.
• What is typically avoided: grains and most heavy foods; many practitioners also avoid root vegetables, while dairy, fruits, and simple liquids are commonly allowed depending on the tradition.
• Differences by tradition: Digambar and Shwetambar communities have subtle differences in the exact rules (what’s allowed, whether water is permitted, how many meals are eaten, etc.). If you want, I can outline the precise rules for eka-upvas in each tradition.

If you’d like, I can provide the exact guidelines for Eka Upvas in Digambar and Shwetambar traditions, with the wording from authentic Jain sources. You can read more here