In any week you can do what ever days work for you – although you should avoid consecutive days if you want it to be sustainable, because it’s too hard for most people to do that. Personally, over the last 5.5 years I’ve always planned on fasting on Monday and Thursday as the norm, but sometimes it’s Monday and Wednesday or Tuesday and Thursday. Sometimes I’ve had to just do it one day a week. I’ve always avoided fasting on Friday and the weekend, and also when on holiday.