the time set for the beginning of a military attack or operation.

the time set for the beginning of any event or action.

a decisive or critical time.

noun

a-day , d-day , h-hour , appointed hour , climax , contingency , countdown , crisis , crisis point , crossroad , crossroads , crunch time , deadline , emergency , exigency , jumping-off point , juncture , moment of truth * , pinch , pivotal moment , start time , strait , target , target day , the time , turning point , turning point * , vital moment , climacteric , exigence , head , pass