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Since you are using positional arguments, django urls are expecting the named group patterns to have the same name as that of the argument (to the view)
So, change
calendar/(\d{4})/(0?[1-9]|1[012])/(?P<user_id>\d+)/
to
calendar/(?P<year>\d{4})/(?P<month>0?[1-9]|1[012])/(?P<user_id>\d+)/
in your urls.py
Source:stackexchange.com