1     public static class DateTimeExtension
 2     {
 3         public const DayOfWeek TheFirstDayOfWeek = DayOfWeek.Monday;
 4 
 5         private static int GetDayOfWeek(this DateTime date, DayOfWeek theFirstDayOfWeek = TheFirstDayOfWeek)
 6         {
 7             return ((int) date.DayOfWeek - (int) theFirstDayOfWeek + 7) % 7;
 8         }
 9 
10         public static bool IsInSameWeek(this DateTime firstDate, DateTime secondDate,
11             DayOfWeek theFirstDayOfWeek = TheFirstDayOfWeek)
12         {
13             var daySpan = (secondDate.Date - firstDate.Date).TotalDays;
14             if (daySpan >= 7d || daySpan <= -7d)
15             {
16                 return false;
17             }
18 
19             var fisrtDayOfWeek = firstDate.GetDayOfWeek(theFirstDayOfWeek);
20             var secondDayOfWeek = secondDate.GetDayOfWeek(theFirstDayOfWeek);
21 
22             return (daySpan >= 0) ^ (fisrtDayOfWeek >= secondDayOfWeek);
23         }
24 
25         public static bool IsLastWeek(this DateTime date, DayOfWeek theFirstDayOfWeek = TheFirstDayOfWeek)
26         {
27             return date.AddDays(7).IsInSameWeek(DateTime.Today, theFirstDayOfWeek);
28         }
29 
30         public static bool IsThisWeek(this DateTime date, DayOfWeek theFirstDayOfWeek = TheFirstDayOfWeek)
31         {
32             return date.IsInSameWeek(DateTime.Today, theFirstDayOfWeek);
33         }
34     }

return (daySpan >= 0) ? fisrtDateOfWeek < secondDateOfWeek : fisrtDateOfWeek >= secondDateOfWeek;