King James Version
For they could not keep it at that time, because the priests had not sanctified themselves sufficiently, neither had the people gathered themselves together to Jerusalem.English Standard Version
for they could not keep it at that time because the priests had not consecrated themselves in sufficient number, nor had the people assembled in Jerusalem--New International Version
They had not been able to celebrate it at the regular time because not enough priests had consecrated themselves and the people had not assembled in Jerusalem.