I go running and have a mileage log. I have a list of non-continuous
dates in column A (i.e. I don't run every day), and mileage figures in
column B.

On a separate part of the worksheet I want to put a summary table that
shows a summary by week. So the first column will be "weekending" date
and the second column mileage for the last seven days. I can't work
out how to put an automatic formula in this second column. Sure, I
could do a manual formula that just sums up the 3 or 4 days in that
week that I ran, but there must be a better way?