‘Now no one at the table knew the reason why he said this to him, for some thought that, because he held the communal purse, Jesus was saying to him, “Buy what we need for the feast”, or else that he should distribute something to the poor'.
This confirms the fact that none of them knew the significance of what had happened. The others merely assumed he had duties to perform. (‘No one', of course, excludes Jesus, and it may exclude the writer. He is speaking of the others). Buying ‘for the feast' refers to the purchases that will need to be made for the remaining days of the feast of Unleavened Bread, and especially for the next day. The 15th of Nisan was always a ‘sabbath' but special concessions were made with regard to preparations for meals on that day. That Judas was thought to be able to purchase at night after the Passover meal demonstrates that food sellers made special provision for providing such goods at that time. In the hot weather food could no be stored for long, especially by visitors. Even John probably saw Jesus as laying a trap for Judas.