You have the option to choose the first day of the week on the week view of the booking form, select on "Set Up" > "Online Booking Forms"
Choose on the "Advanced options" against the form:
This would reflect as a pop up on the right hand side of the page, which should show you all the advanced options you can adjust, including the "First day of week" where you can opt to change that to either days, eg. Sunday, Monday, Tuesday, etc:
Click on "Save" to save any changes made.