When he saw that this pleased the Jews, he proceeded to seize Peter also. This happened during the Feast of Unleavened Bread.The "Feast of Unleavened Bread" sounds awfully strange, but in actuality it is the Jewish holiday that these days is commonly referred to as "Passover". The Passover, you may recall, refers to when the Angel of Death passed over the Israelites in Egypt but killed all the Egyptian first-borns. This finally broke the will of the Pharoah and allowed the Israelites to leave slavery. But in their rush to escape Egypt there was not time to allow the bread to rise, and they had to flee with "unleavened" bread - bread without yeast, basically flat bread.
Acts 12:3
You may remember that Christ also died the day after the Passover meal. This means the events of Acts 12 happened on or about the anniversary of Christ's death and resurrection! The first Passover was freedom from Egypt, the second Passover was about freedom from Sin.