Historical Rules · 29 September 2017
Rule 506.6a
Official source text from the 29 September 2017 Rules release
A spell that states it may be cast “only before (or after) attackers are declared” is referring to the turn-based action of declaring attackers. It may be cast only before (or after) the declare attackers step begins, regardless of whether any attackers are actually declared. (See rule 508.)