Replace use of non-existing method day by date for extra lessons

(cherry picked from commit edb34bc5)

aleksis/apps/alsijil/util/alsijil_helpers.py

@@ -285,8 +285,8 @@ def _generate_dicts_for_events_and_extra_lessons(
             continue
 
         if isinstance(register_object, ExtraLesson):
-            day = date_format(register_object.day)
-            day_sort = register_object.day
+            day = date_format(register_object.date)
+            day_sort = register_object.date
             period = f"{register_object.period.period}."
             period_sort = register_object.period.period
         else: