Get dates on certain weekday between two dates

Question

I receive a start and end DateTime.

From this, I want to create a List<DateTime> of all the dates that are between these two dates, but only on specified weekdays, such as the Monday.

Answer 1

You can generate a list of dates as explained in Create an array or List of all dates between two dates:

public List<DateTime> GetDatesBetween(DateTime start, DateTime end)
{
    var dates = Enumerable.Range(0, 1 + end.Subtract(start).Days)
                          .Select(offset => start.AddDays(offset))
                          .ToList();
    return dates;
}

Now to filter this list to only include weekdays you're interested in is equally trivial by selecting only dates Where() the DayOfWeek property is one of the requested weekdays:

public List<DateTime> GetDatesBetween(DateTime start, DateTime end, params DayOfWeek[] weekdays)
{
    bool allDays = weekdays == null || !weekdays.Any();

    var dates = Enumerable.Range(0, 1 + end.Subtract(start).Days)
                          .Select(offset => start.AddDays(offset))
                          .Where(d => allDays || weekdays.Contains(d.DayOfWeek))
                          .ToList();
    return dates;
}

Answer 2

Bellow function return a List<DateTime> contains all dates from startDate to endDate have given dayOfWeek:

public static List<DateTime> Get_DayofWeek_DatesBetween(DateTime startDate, DateTime endDate, DayOfWeek dayOfWeek)
{
    List<DateTime> list = new List<DateTime>();

    // Total dates in given range. "+ 1" include endDate
    double totalDates = (endDate.Date - startDate.Date).TotalDays + 1;

    // Find first "dayOfWeek" date from startDate
    int i = dayOfWeek - startDate.DayOfWeek;
    if (i < 0) i += 7;

    // Add all "dayOfWeek" dates in given range
    for (int j = i; j < totalDates; j += 7) list.Add(startDate.AddDays(j));

    return list;
}