Can you play an instant when declaring attackers?

[ducttapetitan]

A friend of mine declared and tapped his crusader of odric to attack and then after I killed him with my more powerful crusader of odric, he proceded to play holy day (prevent all combat dammage this turn) which was an instant.  My question is that since he played it after I killed his creature, would it still block the dammage and make it like he never attacked with his card still in play?

[scarsabrex]

He can cast it after blockers are declared but before damage was dealt.