Jews’ passover - See the notes at Matthew 26:2-17. Its being called the Jews’ Passover shows that John wrote this gospel among people who were not Jews, and to whom it was necessary, therefore, to explain their customs.
To purify themselves - This purifying consisted in preparing themselves for the proper observation of the Passover, according to the commands of the law. If any were defiled in any manner by contact with the dead or by any other ceremonial uncleanness, they were required to take the prescribed measures for purification, Leviticus 22:1-6. For want of this, great inconvenience was sometimes experienced. See 2 Chronicles 30:17-18. Different periods were necessary in order to be cleansed from ceremonial pollution. For example, one who had been polluted by the touch of a dead body, of a sepulchre, or by the bones of the dead, was sprinkled on the third and seventh days, by a clean person, with hyssop dipped in water mixed in the ashes of the red heifer. After washing his body and clothes he was then clean. These persons who went up before the Passover were doubtless those who had in some manner been ceremonially polluted.