Does eating a lot on weekends ruin your diet?

Short answer

It is the weekly total, not the weekend itself, that decided the result. When extra weekend eating erased the weekday deficit, weight loss stopped. Weekends planned to stay at maintenance calories did not slow weight loss.

Does eating a lot on weekends ruin your diet?
'Diet 5 days, eat freely 2 days, back to square one' — does this formula hold?
Many people stick to their diet on weekdays and loosen up on weekends. Then the scale is up on Monday, and it feels like the whole week's effort is gone. We checked whether that is really true.
Weight loss really did stop on weekends
Seen in body weight measured every day for 1 year
48 people weighed themselves every morning for 1 year. Before the program, they gained 0.06 kg a day on weekends and nothing on weekdays. The group that cut calories lost weight on weekdays but stopped on weekends, and the exercise group gained 0.08 kg a day on weekends. The cause was eating more on weekends.
How much more do people eat on weekends?
158 kcal on average, most on Saturday
52 people logged their food 1,866 times over 1 year. 75% ate more on weekends, about 158 kcal more per weekend day. Intake was highest on Saturday and lowest on Tuesday. On weekends, alcohol went up and whole grains, fruit and vegetables went down.
But is Monday's weight all fat?
Weight rose and fell by 0.3% every week
In 368 people who weighed themselves for 1 year, weight swung by about 0.3% (252 g) each week, and Monday and Tuesday were the heaviest days. The actual gain over the whole year was a median of 218 g. In 80 people with 4,657 weigh-ins, weight also rose from Saturday and fell from Tuesday.
What mattered was pulling it back on weekdays
People who lost weight came down more on weekdays
In the study of 80 people, the weekend-up, weekday-down pattern was strongest in those who lost or kept their weight, and weakest in those who slowly gained. In 1,421 people, weekday weight also came back down, but over Christmas weight rose 1.35% and had not fully returned months later.
Planned weekends did not slow weight loss
2 days a week at maintenance: no difference in fat loss
27 people dieted for 7 weeks. One group cut calories for 5 days and ate at maintenance for 2 days with extra carbs; the other cut every day. Over the whole week they cut 26% and 21%. Weight dropped 3.2 kg and 3.6 kg, and fat 2.8 kg and 2.3 kg: no difference.
Summary: what weekends can ruin is the weekly total
Eat enough to erase the weekday deficit and loss stops
A 2,500 kcal deficit built by cutting 500 kcal a day on weekdays goes to 0 if you eat 1,250 kcal extra on each weekend day. A higher weekend weight is common and came back down on weekdays. Keeping weekend calories at maintenance and judging your weight week by week is enough.