What is Jack planning for that evening? From chapter eight of Lord of the Flies.
While Ralph and Piggy are trying to figure out how they will go forward, Jack is planning to finally split off for good and trying to figure out how to draw as many of the boys as possible to his side. He tells the boys that he is going to find a pig, kill it, and have a big feast and that they will be safer and more comfortable on his side of the island. Luckily they quickly find a sow, wound her, and are able to track her down and kill her. This provides plenty of meat for the party that Jack has planned.
The other thing that Jack has planned is to find a way to get Piggy's glasses since they have no means to light one otherwise. In order to get them, they plan to attack Ralph and Piggy in the dark and take Piggy's glasses. Once they have the glasses, they also shout to Piggy and Ralph and the others that they ought to come to the party.