Counting Monday and Friday from a Date Range
Your code is already good enough.

static int CountDays(DayOfWeek day, DateTime start, DateTime end)
{
    start = start.Date.AddDays((7 + day - start.DayOfWeek) % 7);
    if (end &lt; start)
        return 0;
    else
        return ((int)(end - start).TotalDays) / 7 + 1;
}