Project(Filter(regex(?date, "^2006-04") ,
               BGP( ?s <http://www.w3.org/2002/12/cal/ical#summary> ?summary .
                    ?s (<http://www.w3.org/2002/12/cal/ical#dtstart>/<http://www.w3.org/2002/12/cal/ical#dateTime>) ?date)),
        {?date, ?summary})