Rashi's Calendar-Based Explanation

Today we discuss two questions:

1. In what formation did the Israelites march through the desert? In other words, what is meant by the phrase (2:17): "As they encamped so they travelled?"

2. The count here is 603,550 adult me. But the same number appears in Shemot 38:26 - a count which would have ben taken at least 6 months previously. Did the population remain static? Did no one die? How do we resolve this mystery?