[Flags]
enum Dates
{ 
    EveryDay = -1,
    EveryMonday = 1,
    EveryTuesday = 2,
    EveryWednesday = 4,
    EveryThursday = 8,
    EveryFriday = 16,
    EverySaturday = 32,
    EverySunday = 64
}

enum Month
{
    January = 1
    // ...
}

struct DateTimeEx
{
    public int Year { get; set; }
    public Month Month { get; set; }
    public Dates Dates { get; set; }

    public override string ToString()
    {
        return Year.ToString() + " " + Month.ToString() + " " + Dates.ToString();
    }

    public List<DateTime> ToDateTimes()
    {
        List<DateTime> dates = new List<DateTime>();
        switch (Dates)
        { 
            case Dates.EveryDay:
                // 把当月所有日期添加到dates
                break;
            case Dates.EveryFriday | Dates.EveryTuesday:
                // 把当月所有周五和周二添加到dates
                break;
            // 列举所有Dates的组合
        }
        return dates;
    }
}