Numbers 6:10
And on the eighth day he shall bring two turtles, or two young pigeons, to the priest, to the door of the tabernacle of the congregation: And on the eighth day he shall bring two turtles, or two young pigeons, to the priest, to the door of the tabernacle of the congregation:
On the eighth day the person was to bring two turtledoves or two young pigeons to the priest at the door of the tabernacle of the congregation. This bringing of birds belonged to the purification requirements laid down for a Nazarite vow. It was part of the offerings made when the vow was being completed and ended.
The sacrifices required for ending a Nazarite vow were costly. In connection with this requirement Paul paid the expenses for four poor believers to complete their vows. He joined them in the seven days of purification.