Intermittent fasting (IF) is an eating pattern that cycles between periods of fasting and eating. It doesn't specify which foods you should eat but rather when you should eat them. In this respect, it's not a diet in the conventional sense but more accurately described as an eating pattern.
To achieve this challenge you are supposed to eat normally for 5 days and fast for 2 days of the week. More likely not consecutive days.
If you start on Monday at 6pm with a last meal, you can eat again on Tuesday at 6pm. For 24 hours you should drink just water.
Adapt this challenge according to your routine. Good luck