-- Database schema
| Air Carriers : Code [ INTEGER ] primary_key , Description [ TEXT ] | Airports : Code [ TEXT ] primary_key , Description [ TEXT ] | Airlines : FL_DATE [ TEXT ] , OP_CARRIER_AIRLINE_ID [ INTEGER ] Airlines.OP_CARRIER_AIRLINE_ID = Air Carriers.Code , TAIL_NUM [ TEXT ] , OP_CARRIER_FL_NUM [ INTEGER ] , ORIGIN_AIRPORT_ID [ INTEGER ] , ORIGIN_AIRPORT_SEQ_ID [ INTEGER ] , ORIGIN_CITY_MARKET_ID [ INTEGER ] , ORIGIN [ TEXT ] Airlines.ORIGIN = Airports.Code , DEST_AIRPORT_ID [ INTEGER ] , DEST_AIRPORT_SEQ_ID [ INTEGER ] , DEST_CITY_MARKET_ID [ INTEGER ] , DEST [ TEXT ] Airlines.DEST = Airports.Code , CRS_DEP_TIME [ INTEGER ] , DEP_TIME [ INTEGER ] , DEP_DELAY [ INTEGER ] , DEP_DELAY_NEW [ INTEGER ] , ARR_TIME [ INTEGER ] , ARR_DELAY [ INTEGER ] , ARR_DELAY_NEW [ INTEGER ] , CANCELLED [ INTEGER ] , CANCELLATION_CODE [ TEXT ] , CRS_ELAPSED_TIME [ INTEGER ] , ACTUAL_ELAPSED_TIME [ INTEGER ] , CARRIER_DELAY [ INTEGER ] , WEATHER_DELAY [ INTEGER ] , NAS_DELAY [ INTEGER ] , SECURITY_DELAY [ INTEGER ] , LATE_AIRCRAFT_DELAY [ INTEGER ] |
-- -- On August 2018, which day had the highest number of cancelled flights due to the most serious reasons in Dallas/Fort Worth International?

SELECT T2.FL_DATE FROM Airports AS T1 INNER JOIN Airlines AS T2 ON T1.Code = T2.ORIGIN WHERE T2.FL_DATE LIKE '2018/8%' AND T1.Description = 'Dallas/Fort Worth, TX: Dallas/Fort Worth International' AND T2.ORIGIN = 'DFW' AND T2.CANCELLED = 1 AND T2.CANCELLATION_CODE = 'A' GROUP BY T2.FL_DATE ORDER BY COUNT(T2.FL_DATE) DESC LIMIT 1;