The Mishna tell us that 39 years have passed since the Exodus and soon the B’nei Yisrael will enter the Promised Land. The story of the Red Heifer is perhaps the most perplexing story in the Torah. Its ashes make impure the pure but also make the pure the impure. This is why it is called a hok, a decree from Hashem which we cannot comprehend but obey because He has commanded it. According to the Mishna, the ceremony of the Red Heifer was only conducted seven times: once by Moshe, once by Ezra, and five times after Ezra. It died with the destruction of the Temple.
The death of Miriam brings to an end the well of water which has followed the B’nei Yisrael through their wanderings in the desert. Like her brothers, she too was not allowed to enter the Land, but she is spoken of as one of the three good leaders because her merit caused the well of water to follow the B’nei Yisrael as long as she lived. We are not told of any mourning time for Miriam because the people were too absorbed with the fact that the well had dried up. However, when Aharon dies later in the sedra the entire assembly wept for him thirty days. This is why we keep thirty days of sheloshim when a close member of our family dies.
Compare this description of the people’s behaviour when Aharon dies with that of their behaviour when Moshe dies. The B’nei Yisrael mourned Moshe thirty days, but we are not told that the whole assembly mourned. Aharon was mourned by the whole assembly because he pursued peace and tried to bring harmony between enemies and friends, between husband and wife. Moshe was not as universally mourned because his was the responsibility to judge and sometimes rebuke the people.
The last part of the sedra is that of Moshe and Aharon striking the rock to bring forth water instead of speaking to it as commanded by Hashem. For this the punishment they were not to enter the Land but to die in the desert. Aharon was punished because he was with Moshe and did not stop the people from building the Golden Calf.
Moshe struck the rock instead of speaking to it. He became angry and called the people ‘rebels’. Rambam, Maimonides, thinks this was his sin while Rashi thinks it was the striking the rock.