Paul acquiesces to James’ plan, and James introduces him to the four men who have taken the Nazarite vow. “My brothers, I wish to join myself to your group and complete your vow with you. When the time is complete, I will also join you in the seven day purification period, after which I will petition the priests at the Temple and pay for all of us to receive our final purification there, consisting of the ritual shaving of our heads and the offering of sacrifices there by the priests. Will you accept me into your group?” The four men agree wholeheartedly, knowing who Paul is and believing that his presence with them will make their sacrifice more worthy in God’s eyes. During this time, many Jews and Jewish believers in Jesus begin to arrive in Jerusalem from all over the world in order to celebrate the festival of Pentecost. They have heard the rumors about Paul and watch him like hawks, waiting for him to do something that they can use to bring charges against him. Paul goes about his daily activities with no sign of apprehension, however. He goes daily to the Temple, where he boldly teaches about Jesus. He stays within the bounds of his vow, abstaining from alcoholic beverages and from shaving or cutting any hair on his body. In short, he acts as any good Pharisaic Jewish man should. When the vow is complete, he and his four companions go through the seven day purification period. They bathe in the mikvah, the ceremonial bathing tub, completely immersing themselves in the water to wash away any impurities. When the seven day period is up, Paul goes to the Temple with the four men to tell the priests that their seven day purification period is over, and they are prepared to offer the required sacrifices and to have their heads shaved. The sacrifices required are a lamb as a burnt offering, a ewe as a sin offering, a ram as a peace offering, a basket of unleavened bread, and grain and drink offerings, which go with the peace offering. Then the priests are to shave their heads and the hair is laid on the fire with the peace offering.
Acts 21:26, https://en.wikipedia.org/wiki/Nazirite