1. Go to Properties > Edit > Go.
2. Scroll down to the section Standard arrival day and tick the days you want to allow as start days for weekly bookings.
Ticking the days a second deselects them again.
3. Click Save at the bottom of the page.
Tip:
If you want to allow e.g. a Monday to be an arrival day for all month of the year, click the All at the top of the Monday column.
If you want to allow all days of the week to be an arrival day for a specific month, click Flexible arrival to the right of the month's row.