If a turn has multiple combat phases, this spell can be cast during any of them as long as it’s after the beginning of that phase’s Declare Blockers Step.

An attacking unblocked creature exists from the time the defending player chooses not to block that creature until the combat phase ends. Curtain of Light doesn’t have any legal targets outside of the combat phase.

Curtain of Light does trigger bushido abilities.

Curtain of Light isn’t particularly effective against creatures with trample. Since the creature with trample has no blocker to assign damage to, all the damage will be assigned to the defending player.