It’s called the 5:2 diet because five days of the week are normal eating days and two days are restricted to 500-600 calories per day.
For the five days, you eat normally and don’t have to think about the calories. While on the remaining two days, you need to reduce the calorie intake, 500 calories for women per day and 600 calories for men.
You can choose any two days of the week. But it’s better to have two separate days with at least one normal day in between them.