static int NumberOfParticularDaysInMonth(int year, int month, DayOfWeek dayOfWeek)
{
    DateTime startDate = new DateTime(year, month, 1);
    int totalDays = startDate.AddMonths(1).Subtract(startDate).Days;

    int answer = Enumerable.Range(1, totalDays)
        .Select(item => new DateTime(year, month, item))
        .Where(date => date.DayOfWeek == dayOfWeek)
        .Count();

    return answer;
}

int numberOfThursdays = NumberOfParticularDaysInMonth(2010, 9, DayOfWeek.Thursday);