start = datetime(2012, 1, 1)
end = datetime(2012, 10, 6)
delta = timedelta(days=1)
d = start
diff = 0
weekend = set([5, 6])
while d <= end:
    if d.weekday() not in weekend:
        diff += 1
d += delta
Цикл по датам, кроме выходных