Learn your history as easter did not begin until the 2nd century.
Passover dates back to 1200 BC.
How did they know what day it was?
Was it not the commemoration of Jesus rising from the dead?
Translating it as "Easter" in the verse in question is not taking it beyond its bounds.
Because Jesus rose from the dead just at the end of Passover; and I would venture to say that peter was not released during Passover but after Passover, right around the same time that Jesus rose from the grave.
But the devil hates this doctrine about Easter; he will do everything in his power to annihilate it from holy scripture.
Because Jesus rising from the dead signifies the devil's defeat on so many levels; therefore he hates the doctrine of Easter (resurrection Sunday).
I personally am edified by the fact that the translators of the kjv used the word Easter instead of Passover.
I would also mention that people who speak Greek in today's world, when they speak of Passover, the Greek word that they use means Easter. So it is not a bad translation in the kjv if you consider the Greek language of today's world.