Expositor's Bible Commentary Commentary Acts 21:26

"Then Paul took the men, and the next day purifying himself with them went into the temple, declaring the fulfilment of the days of purification, until the offering was offered for every one of them." — Acts 21:26 (ASV)

Coming from abroad, Paul would have had to regain ceremonial purity by a seven-day ritual of purification before he could be present at the absolution ceremony of the four Jewish Christians in the Jerusalem temple. In keeping with this ritual, therefore, Paul reported to the priest at the start of his seven days of purification, informing him that he was providing the funds for the offerings of the four impoverished men who had taken Nazirite vows; he undoubtedly returned to the temple at regular prescribed intervals during the week (the third and seventh days) for the appropriate rites.