How could I get the last Monday of every month in C#?

The answers you have received make use of a loop. I am going to add one that is not, and it is also generic to search for any day of the week. The last day of the month is simply calculated, and with whatever day of the week it is, it is calculated which is the previous day of the week that interests us.

This works because the DayOfWeek enumeration assigns an integer to each day of the week, with Sunday being 0 and so on until Saturday is 6.

private DateTime ObtenerUltimoDiaSemanaDelMes(int anio,int mes, System.DayOfWeek dia)
{
    DateTime ultimoDiaMes = new DateTime(anio, mes, DateTime.DaysInMonth(anio, mes));
    int diferencia = ultimoDiaMes.DayOfWeek - dia;
    return diferencia> 0 ? ultimoDiaMes.AddDays(-1 * diferencia) : ultimoDiaMes.AddDays(-1 * (7 + diferencia));
}

The way to call it is as follows:

DateTime ultimoLunes = ObtenerUltimoDiaSemanaDelMes(2019, 12, DayOfWeek.Monday);
DateTime ultimoMiercoles = ObtenerUltimoDiaSemanaDelMes(2019, 12, DayOfWeek.Wednesday);

If you want to know the day number Monday of the month, use the following function:

//Ultimo lunes: retorna valor entero
public int  UltimoLunesDelMes(int anio, int mes)
{
   var ulimoLunesDelMes = new DateTime(anio, mes, DateTime.DaysInMonth(anio,mes));
                
                 //es un loop pequeño sin problemas de performance
                while (ulimoLunesDelMes.DayOfWeek != DayOfWeek.Monday)
                    ulimoLunesDelMes = ulimoLunesDelMes.AddDays(-1);

   return  ulimoLunesDelMes.Day;            
}

//Ejemplo: ultimo lunes de enero del 2019
  var diaUltimo = UltimoLunesDelMes(2019, 1); //28

Cheers

The last Monday of any month would be:

fecha.AddDays(-(double)fecha.DayOfWeek + 1);

When fechait is the last day of the month (and assuming that Monday is valid 1and Sunday is valid 0). Now your problem is to get the last day of the month, which is as simple as going to the first day of the next month and subtracting one day:

var ultimoDiaEnero = (new DateTime(2019, 2, 1)).AddDays(-1);

So, the last Monday of January 2019 was:

var ultimoDiaEnero = (new DateTime(2019, 2, 1)).AddDays(-1);
Console.WriteLine(ultimoDiaEnero.AddDays(-(double)ultimoDiaEnero.DayOfWeek + 1));

The code above shows:

1/28/2019 12:00:00 AM

One way to do this would be to first get the last week of the month you want to get, and then get the Monday of that week like this:

private DateTime GetLastMondayOfMonth(int year, int month)
{
    return FirstDateOfWeekISO8601(year, GetLastWeekOfMonth(year, month));
}

private int GetLastWeekOfMonth(int year, int month)
{
    DateTimeFormatInfo dfi = DateTimeFormatInfo.CurrentInfo;
    //Creamos un DateTime con el año, el mes y los dias totales de ése mes
    DateTime dt = new DateTime(year, month, DateTime.DaysInMonth(year, month));

    Calendar cal = dfi.Calendar;
    //Obtenemos la semana de la fecha creada anteriormente
    return cal.GetWeekOfYear(dt, dfi.CalendarWeekRule,
                                        dfi.FirstDayOfWeek);
}

public static DateTime FirstDateOfWeekISO8601(int year, int weekOfYear)
{
    DateTime jan1 = new DateTime(year, 1, 1);
    int daysOffset = DayOfWeek.Thursday - jan1.DayOfWeek;

    // Usamos el primer Lunes de Enero para obtener la primera semana del año que no sea la semana 52/53/54 del año anterior
    DateTime firstThursday = jan1.AddDays(daysOffset);
    var cal = CultureInfo.CurrentCulture.Calendar;
    int firstWeek = cal.GetWeekOfYear(firstThursday, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);

    var weekNum = weekOfYear;
    // Como estamos agregando días a una fecha en la semana 1, 
    // debemos restar 1 para obtener la fecha correcta para la semana # 1
    if (firstWeek == 1)
    {
        weekNum -= 1;
    }

    // Usar el primer Jueves como semana de inicio garantiza que comencemos en el año correcto
    // luego sumamos el número de semanas multiplicado por días.
    var result = firstThursday.AddDays(weekNum * 7);

    // Restamos 3 dias al Jueves para obtener el Lunes que es el primer dia de la semana segun la ISO8601
    return result.AddDays(-3);
}

what you can do is go back through the days ( for efficiency reasons ) and the first Monday you find is your last Monday of the month:

public DateTime? ObtenerUltimoLunesDelMes(int anyo, int mes)
{
    var diasEnElMes = DateTime.DaysInMonth(anyo, mes);

    for (int dia = diasEnElMes; dia > 0; dia--)
    {
        DateTime actual = new DateTime(anyo, mes, dia);
        if (actual.DayOfWeek == DayOfWeek.Monday)
            return actual;
    }

    return null;
}