The averaging problem
Weight loss depends on your average weekly intake versus your average weekly expenditure -- not on any single day. This means two high-calorie weekend days can offset, or fully cancel, a genuine deficit built across five disciplined weekdays.
A concrete illustration
Five weekdays at a clean 500-kcal daily deficit builds a 2,500-kcal weekly deficit. Two weekend days at just 1,250 kcal over maintenance each erase that entire deficit -- leaving a net weekly result of zero progress, despite five out of seven days technically "on plan."
Why this pattern is so common
Social eating, alcohol, and reduced structure cluster heavily on weekends for most people. It isn't a willpower failure so much as a predictable structural gap -- which is exactly the kind of pattern a probability-based model can quantify rather than moralize about.
What actually helps
Rather than aiming for perfect weekend restriction, many practical approaches spread a smaller, sustainable deficit across all seven days, or bank a modest surplus buffer intentionally into the weekly target so weekend variance doesn't fully erase weekday progress.