Hey Hannah 
Welcome to the community!
This feature has been in the works since July last year I believe. Currently Monzo will try to guess your next payday, and there’s no way to manually alter summary.
More info and the full discussion about summary is below!
https://community.monzo.com/t/summary-feedback-thread/38572/910
You might be able to fiddle around with the settings under the summary tab on the app, but there’s no 100% accurate solution yet.
