Numbers 9

All Israelites including foreigners who sojourned with Israel were commanded to keep the Passover. If there were others who could not keep the Passover because they were ceremonially unclean or because they were away on a journey at the appointed time, a second Passover celebration, the following month, was to be observed.

A cloud, the visible sign of God's presence among Israel, guided Israel's journey. They moved when it moved. They stayed when it stayed.