Any diet plan that lets you eat all the processed sugar and white flour you want on any day is not healthy for you. It should be like this:

M-F: Raw vegetables and low-sugar fruits and nuts
S-S: The same

There is actually a weightloss plan based on only eating gingerbread one day - it is meant to be quite healthy:

Mondays and Tuesdays: raw vegetables
Wednesdays: gingerbread
Thursdays and Fridays: tuna, oysters
Saturdays: eat whatever you want
Sundays: eat nothing, drink hot cocoa.