Sum values for every day of the week

Hi Gurus,

I found many solutions how to SUM every Nth column, but I struggle to find one that matches the start of the sum range.

Assume you have a table of numeric values with products going down and weekdays going across. The first weekday will always change as it references to =TODAY() and not always be Monday for example. How do I find first Monday in the table and then sum every 7th column?

I have put a simple dummy for you to visually understand my issue.

Thanks in advance guys!