There’s a pretty useful visualization of amount spent per day, that’s available for iOS users - the pulse graph.
As you can see, it’s very easy to spot which days you spent the most money on (the big dips in the line - mine are usually bigger than the ones in the video) and when you scroll through your feed of transactions the pointer on the graph scrolls to the same date. So you can see exactly where you spent your money on that date.
But I like the idea of a calendar view. How would you like the design to look? Since there might not be enough space to display the amount spent per day, in a grid displaying a whole month on your phone, perhaps a heat map (red = spent a large amount, green = spent a little) would be a good way to highlight the days you’re looking for?
A better looking version of this -
This would be quite a significant change to the design of the app but even if Monzo decided not to create it, another developer will be able to create an app to display this view, once Monzo’s API is ready.
Just a couple of notes on the pulse graph -
In order to see this graph, you need a full calendar month of transactions & then it will be unlocked on the 3rd of the following month.
If you’re an Android user, this is coming 
it’s next up on the roadmap.