Welcome to the community Anna 
This is a popular request but it’s pretty complex to implement because of the number of features that rely on the fixed timeline, these were Hugo’s initial comments on the idea -
https://community.monzo.com/t/set-start-of-month/1144/4?u=alexs
but…this is being worked on & I’m sure it’ll be available next year
https://community.monzo.com/t/targets-custom-timeframes/6242/2?u=alexs