Fix weekly bymonth

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>