well, if it is not a simple average, I would create my own running total using variables, though you might be able to accomplish this using Running Totals...but they are DBlanks forte.

for variables, you typically have 3 sets of formulas:
initialize/reset, usually in a group header:
shared numbervar x:=0;
"" // this will hide the 0 zero on the report

a summary value, usually in a group footer;
shared numbervar x;
x

so far, pretty easy...now the hardest one, which might be simple
in the detail section, the accumulator
shared numbervar x;
if {table.field} then
x := x +1;
"" //again will hide the running value.

now the question is what to put in the 'if' statement...
I would try the DayOfWeek() function and do something like this:
if DayOfWeek({table.field}) > 1 and DayOfWeek({table.field}) < 7 then
x := x + 1;
""

this should you a count of weekdays, that you can then use to figure out your average.

HTH