Design change / Perf Optimization Suggestion required - response (1) by Fred

For TD13.10 or later, you could use a scalar subquery:
 
SEL count(1) FROM 
( 
  SELECT RTL_ID RetailID,
       CATG_ID CATGID,
       Shipment_Dt ,
       MAX(Catg_Store_Selling_13Wk_Cnt) AS CatgStoreCount
               FROM Mim_tbl.retail_shipment_t
GROUP BY 1,2,3
  )  CATG_STORE_AGG 
WHERE CATG_STORE_AGG.Shipment_DT = (SELECT TMP.YAGO_DT
FROM MIM_TBL.CURR_YAGO_CAL TMP
WHERE TMP.CAL_DT = date'2013-05-11')
 
But why not just put the join inside the GROUP BY, which is what you are trying to achieve anyway:
 
SEL count(1) FROM 
(  SELECT RTL_ID RetailID,
       CATG_ID CATGID,
       Shipment_Dt ,
       MAX(Catg_Store_Selling_13Wk_Cnt) AS CatgStoreCount
               FROM Mim_tbl.retail_shipment_t
INNER JOIN
MIM_TBL.CURR_YAGO_CAL TMP
ON 
CATG_STORE_AGG.SHipment_DT = TMP.CAL_DT
WHERE TMP.CAL_DT = date'2013-05-11'
GROUP BY 1,2,3
  )  CATG_STORE_AGG 
;