Charles Spurgeon Commentary Numbers 9:9-12

"And Jehovah spake unto Moses, saying, Speak unto the children of Israel, saying, If any man of you or of your generations shall be unclean by reason of a dead body, or be on a journey afar off, yet he shall keep the passover unto Jehovah. In the second month on the fourteenth day at even they shall keep it; they shall eat it with unleavened bread and bitter herbs: they shall leave none of it unto the morning, nor break a bone thereof: according to all the statute of the passover they shall keep it." — Numbers 9:9-12 (ASV)

Therefore, provision was made for holding a second Passover, so that persons who were defiled at the first observance might have the opportunity to keep the feast a month later.