Numbers 6:8

All the days of his separation he is holy unto the LORD.

The vow it describes set a person apart as sacred to the Lord for the whole span of the separation, with holiness marking all the days consecrated by it. In Jerusalem this applied to four men who were Christians living under a Nazarite vow and who were nearing the end of their appointed time. Even at that late stage their purification was not yet complete until they presented the required offerings, including sacrifices for purification.

The cost of those closing offerings was heavy, and Paul met the expense for them out of his own means. He then entered with them into the temple observances, sharing in the ceremonies of their purification through the seven days appointed for it.