WITH
/****************************************
** The CTE is same as your query       **
**   except removing ORDER BY clause.  **
****************************************/
 daily_payment AS (
SELECT PRCS_DT 
     , COUNT(*) AS TOTAL 
 FROM  PSSDBAP.PSIPAYV1 PAY 
     , PSSDBAP.PSIBLRV1 BLR 
 WHERE PAY.BLLR_ID = BLR_BLLR_ID 
   AND DATA_SRC_CD = 'P' 
   AND VD_CD       = '2' 
   AND TXN_SRC_CD
               NOT IN ('AV','AS') 
   AND PRCS_DT BETWEEN '05/01/2010'
                   AND '04/30/2011' 
   AND BLR.BLLR_PROD_TYP_CD
               NOT IN ('RT1','RT2') 
   AND PAY.BLLR_ID = '003443' 
 GROUP BY
       PRCS_DT 
)
/****************************************
** End of the CTE.                     **
****************************************/
SELECT m.year_month
     , m.monthly_total
     , d.PRCS_DT AS day_of_max_volume_in_month
     , m.max_day_total
 FROM  (SELECT SUBSTR( CHAR(PRCS_DT , ISO) , 1 , 7 ) AS year_month
             , SUM(TOTAL) AS monthly_total
             , MAX(TOTAL) AS max_day_total
         FROM  daily_payment
         GROUP BY
               SUBSTR( CHAR(PRCS_DT , ISO) , 1 , 7 )
       ) AS m
     , daily_payment AS d
 WHERE m.year_month    = SUBSTR( CHAR(d.PRCS_DT , ISO) , 1 , 7 )
   AND m.max_day_total = d.TOTAL
 ORDER BY
       m.year_month
     , d.PRCS_DT
;