And it turns out for most of us that day is Friday night, at 8pm to be precise according to a new survey by by Lineaslim Effervescent Tablets. We blame takeaways.
The week starts off pretty well, with only two percent of British women saying they're tempted to treat themselves.
It goes pretty much downhill after that, with women finding it difficult to stick to their diet on Wednesdays and Thursdays.
The most challenging day is Friday, with a whopping 22% of women saying that's when their diet fail occurs. Usually in the form of a greasy Chinese or pizza.
Unfortunately Saturdays are no better, with a third of women admitted having three or more bad diet days, often at the weekend.
Dietitian Helen Bond says: "However much pressure we put on ourselves to eat healthy, get fit and lose weight during the week, good intentions often tend to fly out of the window at the weekend."
Well there's always next week, eh?
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