It’s completely fine to occasionally switch your fast days around, if it doesn’t throw your weight loss off-course. For instance, if you have a social outing on Saturday, and that happens to be your fast day, it’s fine to push the fast day back to Sunday. However, just be aware that some people find it difficult to have two fast days back-to-back. Most people find it easy, to start with, to opt for Monday and Wednesday, or Monday and Thursday as their two fast days. Alternatively, if you’re fasting for three days, you can do Sunday, Tuesday and Thursday, or Monday, Wednesday and Friday. Find what works for you.