One day shows a negative margin. Is that a bug?

Usually not. A quiet day still carries its share of your fixed costs. And a day with many refunds from a busy week before can give back more than it earned. Both are true facts about that day, not errors.

These are the usual reasons. Overhead is spread over every day, so a Sunday with few orders still pays its share of rent and salaries. Refunds land on the day they happen, not on the day of the order. Ad spend lands on the day it was spent, whatever day its orders came. And a one-off cost, like a repair, lands whole on the day you booked it.

Check whether the day is negative at CM2, before ads and fixed costs. If CM2 is negative, the orders themselves lost money. That points to a price or cost problem, not the calendar. If CM2 is positive and only CM3 or EBITDA drop below zero, the day simply did not cover its share of ads or fixed costs.

Do not judge one day on its own. Compare it with the same weekday a week before, and with the month so far. The Daily profit chart on the Overview draws loss days below the zero line for exactly this. The post anatomy of a losing day walks through such a day, line by line.