fix 4/5 week dts timing bug

Revert "fix 4/5 week dts timing bug"

This reverts commit b7bbc89.

Fix issues with 5 week DST transition months

src/Ical.Net.CoreUnitTests/VTimeZoneTest.cs

@@ -153,6 +153,21 @@ public void VTimeZoneAmericaLosAngelesShouldSerializeProperly()
             //Assert.IsTrue(serialized.Contains("RDATE:19600424T010000"), "RDATE:19600424T010000 was not serialized");  // NodaTime doesn't match with what tzurl has
         }
 
+        [Test, Category("VTimeZone")]
+        public void VTimeZoneEuropeOsloShouldSerializeProperly()
+        {
+            var iCal = CreateTestCalendar("Europe/Oslo");
+            var serializer = new CalendarSerializer();
+            var serialized = serializer.SerializeToString(iCal);
+
+            Assert.IsTrue(serialized.Contains("TZID:Europe/Oslo"), "Time zone not found in serialization");
+            Assert.IsTrue(serialized.Contains("BEGIN:STANDARD"), "The standard timezone info was not serialized");
+            Assert.IsTrue(serialized.Contains("BEGIN:DAYLIGHT"), "The daylight timezone info was not serialized");
+            Assert.IsTrue(serialized.Contains("BYDAY=-1SU;BYMONTH=3"), "BYDAY=-1SU;BYMONTH=3 was not serialized");
+            Assert.IsTrue(serialized.Contains("BYDAY=-1SU;BYMONTH=10"), "BYDAY=-1SU;BYMONTH=10 was not serialized");
+
+        }
+
         [Test, Category("VTimeZone")]
         public void VTimeZoneAmericaAnchorageShouldSerializeProperly()
         {

src/Ical.Net/CalendarComponents/VTimeZone.cs

@@ -254,9 +254,12 @@ public IntervalRecurrencePattern(ZoneInterval interval)
 
                 var date = interval.IsoLocalStart.ToDateTimeUnspecified();
                 var weekday = date.DayOfWeek;
-                var num = DateUtil.WeekOfMonth(date);
+                var weekNumber = DateUtil.WeekOfMonth(date);
 
-                ByDay.Add(num != 5 ? new WeekDay(weekday, num) : new WeekDay(weekday, -1));
+                if (weekNumber >= 4)
+                    ByDay.Add(new WeekDay(weekday, -1)); // Almost certainly likely last X-day of month. Avoid issues with 4/5 sundays in different year/months. Ideally, use the nodazone tz database rule for this interval instead.
+                else
+                    ByDay.Add(new WeekDay(weekday, weekNumber));
             }
         }