“now on the next day.” This is the morning of Nisan 15 (Nisan 15 had started at sunset the night before). The Passover sacrifice is killed in the late afternoon on Nisan 14 but eaten after sunset. Since sunset starts the next day, the Passover meal actually ends up being eaten on the next day, Nisan 15, which is the first day of the seven-day Feast of Unleavened Bread (Exod. 12:6-19). In spite of the fact it was a Sabbath day, the Pharisees and chief priests were so filled with trepidation about Jesus that they went to Pilate and requested a guard to keep the tomb secure.
Pilate’s answer, “You have a watch,” or as it is in some versions, “You have a guard,” has sometimes been misunderstood to mean that the Priests already had the Temple police, so they should use them. That is not correct. Pilate gave permission to the priests to requisition a detachment of Roman guards, which is why the guards would have been in trouble if the governor heard that the body of Jesus had been stolen while they were guarding it (cp. Matt. 28:12-15).
“gathered together.” This gathering would not have involved every member of the Sanhedrin, but only a select group. Also, they would not have met together somewhere and then marched as a group to Pilate because that would have attracted too much attention, and it was a Sabbath day. Instead, they would have communicated their purpose quietly, and then gone as individuals to Pilate, gathering together as a group once they were in his presence.