WWII turning point (4)
'wwii turning point' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for d-day that I've seen before include "June six, nineteen fourty-four" , "Allied landings date" , "Launch date" , "World War Two Normandy landings date" , "Tricky landings" .)
(Another definition for dday that I've seen is " critical time for action".)