diff --git a/plan/libs/models/kcalendar/kdatetable.cpp b/plan/libs/models/kcalendar/kdatetable.cpp
--- a/plan/libs/models/kcalendar/kdatetable.cpp
+++ b/plan/libs/models/kcalendar/kdatetable.cpp
@@ -50,6 +50,7 @@
 public:
     KDateTablePrivate(KDateTable *q): q(q)
     {
+        m_weekDayFirstOfMonth = 1;
         m_popupMenuEnabled = false;
         m_selectionmode = KDateTable::SingleSelection;
         m_paintweeknumbers = false;
@@ -123,9 +124,9 @@
 {
     if (!date.isValid()) {
         debugPlan << "KDateTable ctor: WARNING: Given date is invalid, using current date.";
-        initWidget(QDate::currentDate()); // this initializes m_weekDayFirstOfMonth, m_numDaysThisMonth, numDaysPrevMonth
+        initWidget(QDate::currentDate());
     } else {
-        initWidget(date); // this initializes m_weekDayFirstOfMonth, m_numDaysThisMonth, numDaysPrevMonth
+        initWidget(date);
     }
 }
 
@@ -832,6 +833,8 @@
         return false;
     }
 
+    d->m_weekDayFirstOfMonth = QDate(date_.year(), date_.month(), 1).dayOfWeek();
+
     if (d->m_date != date_) {
         const QDate oldDate = d->m_date;
         d->m_date = date_;