Welcome to the community 
This is a pretty popular request, here’s Hugo’s initial comments on the idea -
https://community.monzo.com/t/set-start-of-month/1144/4?u=alexs
& the latest update -
https://community.monzo.com/t/targets-custom-timeframes/6242/2?u=alexs
You can check out a list of popular feature ideas here -
https://community.monzo.com/t/popular-feature-ideas-bulb-open-wiki/6758