こういう嘘くさいメソッドを捏造してやればおｋ

public static int 第nx曜日の日付を調べる(int n, DayOfWeek x, DayOfWeek firstDay)
{
int firstDateX = x - firstDay + 1;
if (firstDateX <= 0)
firstDateX += 7;
return firstDateX + 7 * (n - 1);
}