Not directly money related but it would be helpful to be warned a day or so in advance when there is a public holiday in the country you are in. Last week I was in Spain where 1 November is All Saints Day. The bus from the hotel didn’t run and shops were shut. A warning in advance would help you plan your activities. The ATMs still worked of course but knowing say that supermarkets may be shut and public transport limited can be really useful.
I’m not that convinced this is something Monzo should take on, personally. While it would be a handy feature, many Maps and calendar apps already show this kind of information. For example, while I was in Japan, Google Maps warned me that culture day might affect opening times of places and warned of areas that might become overcrowded. I also believe Google Trips also has similar warnings.
Open related question: Is there any open or free data feed of all these worldwide holidays that Monzo could potentially subscribe to? I know you could subscribe to the Google or Apple default calendars for each country but that doesn’t scale too well and only has limited country support.
Time and Date have a paid API: https://www.timeanddate.com/services/api/holiday-api.html