They were still in the wilderness of Sinai. They had not yet moved along from Mt. Sinai. Now, as it happened, some of the people were unclean according to the rules that had been set by God and Moses concerning the touching of a corpse. It seemed to them that they were at an impasse. How could they obey God if there were two commandments that contradicted each other? They were required to observe the Passover and they were required not to do so... due to their uncleanness. So, God made a provision for individuals under those kinds of circumstances. They were required to celebrate the Passover one month later in more or less a smaller and more private manner (Number 9:10-11). However, except for those who were unclean (or traveling in a distant region), there were not to be any delays or postponements in the observance of this ritual. So, April 14th was Passover and May 14th was the makeup date. By God's injunction, failure to celebrate Passover was supposed to be a fatal error (Numbers 9:13). Of course, for us, the full meaning of Passover is just as important.