God had specified that the firstborn children of each of the families and the firstborn of their livestock would be His. Instead of taking these firstborn children and livestock from the children of Israel, God chose to use the Levites and their livestock as substitutes. Before the Levites could begin their service in the tabernacle, it was necessary that they would be made ceremonially clean. God gave instructions for that cleansing to Moses and it was performed. Provisions were made to allow Levites from twenty-five years of age to also assist in the service of the tabernacle.
The fourteenth day of the first month of the year and succeeding years was the time for remembering the Passover. God had saved the children of Israel from the tenth plague of death of the firstborn against Egypt and had delivered them from Pharaoh. No one who was unclean could participate in this great occasion. However, provisions were made for someone who had touched a dead body or was traveling and unable to attend to celebrate the Passover one month later.
God was continually present with the Israelites in the form of a cloud over the tabernacle. The cloud appeared fiery at night to make it more visible to the people. If the cloud moved, it was the order for the Israelites to break camp and to follow it. When it was time to stop traveling, the cloud would stand still above the tabernacle.