The 5 days 'on diet' is supposed to be 5 consecutive days (as mentioned, mon-fri), which fits in with the usual working week, where a lot of people avoid getting drunk/partying/staying out late, and leave that until the weekend. So by 'eating clean' for those 5 days, it kind of means you can 'reward' yourself with 2 days 'off diet'.
If I'm completely honest with you, I really don't think this diet is best suited to fat loss. If you're someone who has a lot of weight to lose, then having 2 days 'off diet' isn't the best thing. Those two days will be filled with eating and drinking what you want, which can very quickly add up to a LOT of calories. Also, those 2 days 'off diet' can easily become 3 - you will at some point think "bringing my 'off diet' forward one day couldn't do any harm, right?".
The first 5 days 'on diet', should be filled with healthy, lean, clean foods - lots of vegetables, lean protein, some complex carbs, plenty of water and hitting the gym hard. Obviously you will have to work out your calorie intake by your calorie expenditure and your overall goal - fat loss or muscle gain. As you will be training hard in the gym, you will need carbs, so aim for around 30% of your diet coming from protein, 20% healthy fats and 50% from complex carbs,
This 5 days 'on diet' will not be easy. You need to be strict with your diet and train dam hard in order to 'earn' your 2 days off. Similar to a high-stress job - work your butt off mon-fri and have the weekend to relax. You want to keep your workouts high intensity, whether your lifting heavy weights, cross-fit, strength training, old school cardio or any other kind of workout, you need to smash it.
When it comes to your 'off diet' days, that doesn't necessarily mean you can eat what you want and how much you want - it simply means you can relax with your calorie intake. You still want to eat protein, but you don't have to 'eat clean'. Don't go crazy with your meals though - no gigantic garbage take-away deliveries!
On the surface of things, the 5:2 diet doesn't seem to bad, but it is actually very hard to stick to. I haven't purposely done it, but have accidentally. I've had weekends where I've 'let myself go' and gone off diet, but I haven't liked it. The thing with fitness, is that you can become (healthily) addicted, and once you get into the routine, you could very easily not like 'eating what you want' on the weekends. The times where I've gone off of my diet on weekends, I've felt lazy, fat, sluggish, and wish I didn't do it! Maybe that's what this diet is about, guilt tripping you into realising eating junk is bad? who knows...
That's my take on it, everyone will have their opinion and I'm sure there are guys and girls out there who are in great shape who live by the 5: 2 method, but each to their own.
Lee Gregory Fitness