diff --git a/calendar/lib.php b/calendar/lib.php
index 45c1785e2cd..0a8408f3641 100644
--- a/calendar/lib.php
+++ b/calendar/lib.php
@@ -249,10 +249,6 @@ class calendar_event {
         }
 
         $this->properties = $data;
-
-        if (empty($data->context)) {
-            $this->properties->context = $this->calculate_context();
-        }
     }
 
     /**
@@ -343,6 +339,24 @@ class calendar_event {
         return $context;
     }
 
+    /**
+     * Returns the context for this event. The context is calculated
+     * the first time is is requested and then stored in a member
+     * variable to be returned each subsequent time.
+     *
+     * This is a magical getter function that will be called when
+     * ever the context property is accessed, e.g. $event->context.
+     *
+     * @return context
+     */
+    protected function get_context() {
+        if (!isset($this->properties->context)) {
+            $this->properties->context = $this->calculate_context();
+        }
+
+        return $this->properties->context;
+    }
+
     /**
      * Returns an array of editoroptions for this event.
      *
@@ -367,7 +381,7 @@ class calendar_event {
             // Check if we have already resolved the context for this event.
             if ($this->editorcontext === null) {
                 // Switch on the event type to decide upon the appropriate context to use for this event.
-                $this->editorcontext = $this->properties->context;
+                $this->editorcontext = $this->get_context();
                 if (!calendar_is_valid_eventtype($this->properties->eventtype)) {
                     return clean_text($this->properties->description, $this->properties->format);
                 }
@@ -433,7 +447,7 @@ class calendar_event {
 
         // Prepare event data.
         $eventargs = array(
-            'context' => $this->properties->context,
+            'context' => $this->get_context(),
             'objectid' => $this->properties->id,
             'other' => array(
                 'repeatid' => empty($this->properties->repeatid) ? 0 : $this->properties->repeatid,
@@ -485,7 +499,7 @@ class calendar_event {
                 // were set when calculate_context() was called from the constructor.
                 if ($usingeditor) {
                     $this->properties->context = $this->calculate_context();
-                    $this->editorcontext = $this->properties->context;
+                    $this->editorcontext = $this->get_context();
                 }
 
                 $editor = $this->properties->description;
@@ -512,7 +526,7 @@ class calendar_event {
 
             // Log the event entry.
             $eventargs['objectid'] = $this->properties->id;
-            $eventargs['context'] = $this->properties->context;
+            $eventargs['context'] = $this->get_context();
             $event = \core\event\calendar_event_created::create($eventargs);
             $event->trigger();
 
@@ -681,7 +695,7 @@ class calendar_event {
 
         // Trigger an event for the delete action.
         $eventargs = array(
-            'context' => $this->properties->context,
+            'context' => $this->get_context(),
             'objectid' => $this->properties->id,
             'other' => array(
                 'repeatid' => empty($this->properties->repeatid) ? 0 : $this->properties->repeatid,
@@ -715,7 +729,7 @@ class calendar_event {
 
         // If the editor context hasn't already been set then set it now.
         if ($this->editorcontext === null) {
-            $this->editorcontext = $this->properties->context;
+            $this->editorcontext = $this->get_context();
         }
 
         // If the context has been set delete all associated files.
@@ -774,10 +788,10 @@ class calendar_event {
 
                 if ($properties->eventtype === 'site') {
                     // Site context.
-                    $this->editorcontext = $this->properties->context;
+                    $this->editorcontext = $this->get_context();
                 } else if ($properties->eventtype === 'user') {
                     // User context.
-                    $this->editorcontext = $this->properties->context;
+                    $this->editorcontext = $this->get_context();
                 } else if ($properties->eventtype === 'group' || $properties->eventtype === 'course') {
                     // First check the course is valid.
                     $course = $DB->get_record('course', array('id' => $properties->courseid));
@@ -785,7 +799,7 @@ class calendar_event {
                         print_error('invalidcourse');
                     }
                     // Course context.
-                    $this->editorcontext = $this->properties->context;
+                    $this->editorcontext = $this->get_context();
                     // We have a course and are within the course context so we had
                     // better use the courses max bytes value.
                     $this->editoroptions['maxbytes'] = $course->maxbytes;
@@ -793,7 +807,7 @@ class calendar_event {
                     // First check the course is valid.
                     \coursecat::get($properties->categoryid, MUST_EXIST, true);
                     // Course context.
-                    $this->editorcontext = $this->properties->context;
+                    $this->editorcontext = $this->get_context();
                     // We have a course and are within the course context so we had
                     // better use the courses max bytes value.
                     $this->editoroptions['maxbytes'] = $course->maxbytes;
@@ -869,7 +883,7 @@ class calendar_event {
 
         // Prepare event data.
         $eventargs = array(
-            'context' => $this->properties->context,
+            'context' => $this->get_context(),
             'objectid' => $this->properties->id,
             'other' => array(
                 'repeatid' => empty($this->properties->repeatid) ? 0 : $this->properties->repeatid,
@@ -939,7 +953,7 @@ class calendar_event {
 
         if ($this->editorcontext === null) {
             // Switch on the event type to decide upon the appropriate context to use for this event.
-            $this->editorcontext = $this->properties->context;
+            $this->editorcontext = $this->get_context();
 
             if (!calendar_is_valid_eventtype($this->properties->eventtype)) {
                 // We don't have a context here, do a normal format_text.