“Aharon the priest ascended Hor Hahar and died there in the fortieth year... in the fifth month on the first of the month” (Bamidbar 33:38). It is on rare occasions that the Torah actually dates events recorded therein. Even the giving of the Torah at Sinai has no biblical date associated with it. Birthdays, anniversaries and yahrzeits are of little interest to the Bible. The tradition that Moshe dies on the 7th of Adar is one deduced, without 100% accuracy, from the narrative of Sefer Yehoshua. The day of death for our imahot and avot are nowhere to be found—the deaths of Leah and Rivka are not even recorded in the biblical text.
Thus, it is somewhat unusual that the Torah tells us that Aharon died on the “first of the fifth month”, Rosh Chodesh Av. In fact, his is the only “yahrzeit” mentioned in the Torah. Doubly strange is why this is mentioned in parshat Massei and not in parshat Chukat, where Aharon’s death is actually recorded.
Aharon’s death unleashed grief never before or after seen amongst the Jewish people. “They wept for Aharon thirty days, the entire house of Israel” (Bamidbar 20:29), surpassing the mourning displayed at the death of Moshe.
The fact that the date of Aharon’s death was recorded in the Bible means that it must have historical significance. “Many prophets arose for the Jewish people, double the number who left Egypt. However, prophecy that was needed for future generations was written down, but that which was not needed, was not written” (Megillah 14a). Whatever prophecy we have—and the Bible is a book of prophecy—is because the prophecy relates to current events. The life events of Avraham and Sarah and other biblical figures interested the Torah little, hence so little is recorded about their personal lives. Only that which is necessary for our moral edification merits mention in the Torah. The stories, for example, of Yaakov and Eisav are recorded because they reflect the ongoing struggle between the descendants of Yaakov, the B’nei Yisrael, and those of his antagonist Eisav.
The death of Aharon is recorded less for the great personal loss than for the great communal loss for us today. Aharon Hakohen is known in our tradition as the ohev shalom v’rodef shalom, the lover and pursuer of peace. With his death, who would ensure communal peace? Who would, rightly or wrongly (a question debated by the commentaries), risk allowing the idolatry of the golden calf to take place in a (failed) quest to prevent strife?
Aharon’s death is recorded in parshat Chukat, soon after the incident of the hitting of the rock at Mei Merivah. As part of the story of the journey in the desert, the date of his death is irrelevant. Yet as the Torah records the list of places we encamped during our aimless wandering in the desert, the death of Aharon is mentioned again, with the Torah adding the date of death. At this juncture, mention of his death is a call to remember the legacy of Aharon, the yearning for peace at almost all costs. If we are remiss in following in Aharon’s path, we might have to wander in exile again for many years. And here, the date is crucial.
“When Av enters, we diminish our joy” (Taanit 29a). The hatred or, even more tragically, the indifference that has been our lot for the last 2,000 years is testimony to the painful loss of Aharon. He did not die only once, thousands of years ago—he dies every year. “Every generation that does not merit having rebuilt the Temple is as if they had destroyed it” (Talmud Yerushalmi Yoma 1:1).
While the weekly Torah reading is of much later origin, it is not coincidental that the dating of Aharon’s death is always read during the week in which Rosh Chodesh Av occurs. “The righteous in their death are considered alive.” We can revive Aharon by following in his footsteps of pursuing peace. If we do so, we will merit the fulfillment of the prophecy of Zechariah (8:19), “The fast of the fifth [month, i.e., the 9th of Av] will be for joy and gladness, love, truth and peace”.
 While this admonition is of rabbinic origin, our Sages already noted that the meraglim returned on the ninth of Av. In a sad example of how biblical events foreshadow future events, maaseh avot siman lebanim, G-d proclaims, “you cried needlessly, I will establish for you crying for generations” (Taanit 29a). The death of Aharon, the ohev shalom v'rodef shalom, more than the death of any other is cause for much crying.