using System;

using System.Globalization;

public class Program

{

    public static int GetIso8601WeekOfYear(DateTime time)

    {

        DayOfWeek day = CultureInfo.InvariantCulture.Calendar.GetDayOfWeek(time);

        if (day >= DayOfWeek.Monday && day <= DayOfWeek.Wednesday)

        {

            time = time.AddDays(3);

        }

        return CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);

    } 

    public static int GetNETWeekOfYear(DateTime time) {

        return CultureInfo.InvariantCulture.Calendar.GetWeekOfYear(time, CalendarWeekRule.FirstFourDayWeek, DayOfWeek.Monday);

    }

    public static void Main()

    {

        var startDate = new DateTime(2018, 12, 30);

        var endDate = new DateTime(2019, 1, 20);

​

        int diff = (7 + (startDate.DayOfWeek - DayOfWeek.Monday)) % 7;

        var weekStartDate = startDate.AddDays(-1 * diff).Date;

        var i = 1;

        var weekEndDate = DateTime.MinValue;    

        while(weekEndDate < endDate) {

            weekEndDate = weekStartDate.AddDays(6);

            var shownStartDate = weekStartDate < startDate ? startDate : weekStartDate;

            var shownEndDate = weekEndDate > endDate ? endDate : weekEndDate;

            Console.WriteLine($"{i++}. Week {GetIso8601WeekOfYear(shownStartDate)} ({GetNETWeekOfYear(shownStartDate)}): {shownStartDate:dd MMMM yyyy} - {shownEndDate:dd MMMM yyyy}");

            weekStartDate = weekStartDate.AddDays(7);

        }

    }

}