added EventDetailView in urls.py

widget_princessgabi/widget_calendar/urls.py

 from django.urls import path
 
 from . import views
+from .views import (EventDetailView)
 
 urlpatterns = [
     path('', views.Calendar, name=''),
+    path('events/<int:pk>/details/', EventDetailView.as_view(), name='event-details'),
 ]
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