When you specify the start date for a spending period, there should be some sort of toggle/checkbox to enable selecting “working days only”.
When this option is checked, and the start date specified falls on a non-working day, the start date should be moved to the closest working day before the specified period start, and affected spending periods should be automatically adjusted.
For example, I get paid on the 23rd of the month or, if it falls on a weekend/bank holiday, the last working day before the 23rd. So if, like June this year, the 23rd was a Saturday, my pay and a lot of the manual transfers I make occurred on the 22nd. This means the monthly spending report is completely off because according to Monzo, I had two wage packets in one spending period. The spending report for most of July will be similarly skewed.