Either use Avg() function or aggregate your measure and divide by e.g. number of distinct dates in your period:
Average – Which average?
If you want to limit the data to the present week, either use a conditional function or set analysis:
=Avg( If( DateField >= Weekstart(Today()) and DateField <= Weekend(Today()), ValueField))
or
Avg({<DateField = {">=$(=Date(Weekstart(Today())))<=$(=Date(Weekend(Today())))"} >} ValueField)
See also
A Primer on Set Analysis
Dates in Set Analysis
