You can get to this information in the iOS app too but it takes slightly longer & your options are more limited than what Richard’s described for the Android app (who would have thought that could happen?! 
).
Tap into a transaction > tap the any of the three panels in the ‘YOUR MERCHANT NAME HISTORY’ list > & use the search bar at the top of the window, to filter your search. You can also search for the merchant’s name & then apply the date filter.
Your options for the search by date are:
This / Last: Year, Quarter, Month, Week, Weekend
which is obviously slightly limited but Hugo has shared a preview of the upcoming updates for the search feature, which will allow you to apply more filters, including specifying date ranges -
https://community.monzo.com/t/not-equal-to-search-modifier/5935/5?u=alexs