From Chabad.org: This week’s reading, Bamidbar, begins the Book of Numbers, the fourth of the Five Books of Moshe. This book of the Torah opens on the first of Iyar, one month after the inauguration of the Tabernacle, and several weeks before the Jews will depart from Mount Sinai and begin their journey to the Holy Land. In this week’s portion the Israelites and the Tribe of Levi are counted separately. G‑d instructs the Israelites on how to camp in the desert, surrounding the Tabernacle. The Levites are informed the procedure for dismantling the Tabernacle before traveling.

In the first Aliya G‑d commands Moshe to count all Jewish men of military age. G‑d names one member of each tribe as the nasi, leader, of the tribe. Each nasi will assist Moshe and Aaron in taking a census of his tribe. An additional objective of this census was to establish the tribal lineage of every Jew.