Specify day of week for monthly backups in WHM

I'd like the ability to specify the day of the week for monthly backups in WHM to run, e.g. the first Sunday of every month, or, to specify my own '4 week interval' (which I'd start on a Sunday).

The goal being to run backups on a Sunday during 'off-peak'.

My server performance is presently being impacted inconveniently by backups mid-week (peak traffic) when they run on 1st of each month and I have no control over the day.