As a form of IF - Intermittent Fasting, basically the plan is 5 days of "regular" eating - these are feast days and 2 non-consecutive "fast" days which allows women 500 calories and men 600 calories. Easy peasy yapaneesy! The nice thing is that when you get through your 500 calorie day and a feast day comes you're not all that hungry for junk food. Too, if you are doing this for weightloss try not to over eat on feast days. Stick to around 2000 calories or your TDEE. However, I haven't found it very detrimental to the weightloss if you do go over on feast days.
You can plan your fast days when ever you want, Monday & Thursday, Tuesday & Thursday, Monday & Friday. Its up to you. Another beauty of the plan is that it allows social flexibility. If your scheduled fast day doesn't fit into your social schedule (birthday party, social outing, etc), you can just move your fast day to the next day or another day of the week. It doesn't have to be the same days each week.
The Health Benefits:
People are seeing lower blood pressure, lower cholesterol, blood sugar regulation, more energy, improved mood and clearer skin. Increased lifespan and protection against age-related diseases such as cancer, dementia and Alzheimer’s are also benefits of this intermittent fasting. So while some do it for weightloss, some solely do it for the health reasons.
Dr. Michael Mosely has written a book on the diet and he is known as the 5:2 guru. He has also done a documentary which can be found on Youtube.