Numbers 9

"Speak to the children of Israel, saying: ‘If anyone of you or your posterity is unclean because of a corpse, or is far away on a journey, he may still keep the Lord’s Passover.  On the fourteenth day of the second month, at twilight, they may keep it. They shall eat it with unleavened bread and bitter herbs." vv.10-11

Read chapter 9
Here is the first passover celebrated since the exodus out of Egypt.  From the first command to celebrate the Passover in Exodus 12 we learn that they weren't required to keep the Passover until they were in the promised land, "It will come to pass when you come to the land which the Lord will give you, just as He promised, that you shall keep this service" (v.25).  The passover is brought up in this specific passage to reference a specific matter concerning of keeping the passover.  Someone who was unclean was not allowed to come in to the camp to or be around others until they were clean again based on sacrifices and offerings concerning what made them unclean.  They desire to keep the Passover and not just have an out to not have to partake of the religious festival, so we see the Lord accommodate this and that it is for His glory, not for man's.  It then becomes a month later, an appointed time, when those that were unclean during the Passover on the 14th day of the first month can celebrate it on the 14th day of the second month.  
How great a God we have seen in the past couple chapters of specific ordinances for the betterment of His people and for His glory.  This doesn't make Him a changing God, but a compassionate and just God.  How can you adjust to changes and still bring the glory to God?