Fix weekly bymonth (#880)

Weekly rules that include BYMONTH were only including weeks that
start inside the given months. This fix checks the end of the week
also to see if any part of the week is inside the given months.

The date will not always be the start of a week, since the
reference date is first, but extra months are filtered out later.

Fixes #879

Author: maknapp

Ical.Net.Tests/RecurrenceTests_From_Issues.cs

@@ -10,6 +10,7 @@
 using System.Linq;
 using Ical.Net.CalendarComponents;
 using Ical.Net.DataTypes;
+using Ical.Net.Evaluation;
 using NUnit.Framework;
 
 namespace Ical.Net.Tests;
@@ -513,4 +514,21 @@ public void Except_Tuesday_Thursday_Saturday_Sunday()
             }
         });
     }
+
+    [Test]
+    public void Weekly_ByDay_ByMonth()
+    {
+        // Bug #879
+
+        var rp = new RecurrencePattern("FREQ=WEEKLY;BYDAY=SU,MO,TU,WE,TH,FR;BYMONTH=5,6,7,8,9");
+        var rpe = new RecurrencePatternEvaluator(rp);
+
+        var referenceDate = new CalDateTime(2025, 11, 07, 0, 0, 0, "Central Standard Time");
+        var recurringPeriods = rpe.Evaluate(referenceDate, null, null);
+
+        var result = recurringPeriods.FirstOrDefault()?.StartTime;
+
+        var expected = new CalDateTime(2026, 05, 01, 0, 0, 0, "Central Standard Time");
+        Assert.That(result, Is.EqualTo(expected));
+    }
 }

Ical.Net/Evaluation/RecurrencePatternEvaluator.cs

@@ -308,7 +308,18 @@ private static IEnumerable<CalDateTime> GetMonthVariants(IEnumerable<CalDateTime
         }
 
         // Limit behavior
-        return dates.Where(date => pattern.ByMonth.Contains(date.Month));
+        if (pattern.Frequency == FrequencyType.Weekly)
+        {
+            // The dates here represent weeks, with each date being the
+            // start of a week except for the initial reference date.
+            // Return weeks that have any day within BYMONTH.
+            return dates.Where(date => pattern.ByMonth.Contains(date.Month)
+                || pattern.ByMonth.Contains(date.AddDays(6).Month));
+        }
+        else
+        {
+            return dates.Where(date => pattern.ByMonth.Contains(date.Month));
+        }
     }
 
     /// <summary>