Why is Aaron HaCohen's Yahrtzeit the only one mentioned in the Torah? None of the Avot or the Imahot were accorded such an honor and not even Moshe Rabbenu. Only the date of the death of Aaron is mentioned. Why?

Aaron's Yahrtzeit is the first day of the month of Av, and the Jewish calendar is structured so that Parshat Masei which records Aaron's death is read before Tisha B'Av. Why?

The Mishnah in Avot states that "Aaron loved peace and pursued peace". With his passing, disputes and divisiveness increased. The Talmud in Yoma 9 attributes the destruction of the Second Temple to "baseless hatred" ,Sinat Chinam,  among Jews.
The destruction of both Temples was on Tisha B'Av.

The Yahrtzeit of Aaron alone emphasizes the critical importance of his character traits, which are "loving peace, pursuing peace" and avoiding Machlokot. Thus, his Yahrtzeit is in Parshat Masei, which is read before Tisha B'Av, to teach us that Aaron's exemplary behavior is the Tikun of SINAT CHINAM, which caused the tragedies of  Tisha B'Av.

The prophet Zecharia states that the Fast of the 17th of Tammuz, the Fast of Tisha B'Av, the Fast of Tzom Gedalia, and the Fast of the 10th of Tevet, will be for the House of the Jews for gladness and joyous festivals. This prophecy will be fulfilled in the Messianic Era.

Why is it that in the time of Moshiach these former fast days, commemorating the Temple's  destruction, will become joyous festivals and not just ordinary days?
The answer is that these tragic days will assume their original intent.

The 17th of Tammuz was the date of the giving of the Tablets of the Ten Commandments that was supposed to be a day of great joy. However, that is the very day that the Jews worshipped the Golden Calf, and Moshe smashed G-d's Holy Tablets. Moshe had to re-ascend Mt. Sinai and plead for G-d's forgiveness for the Jewish People. The Second Tablets were not given until Yom Kippur. When we do Teshuva and merit the arrival of Moshiach, we will have fully rectified the Sin of the Golden Calf. Thus, the 17th of Tammuz will assume its original character, a day of festive joy for receiving the First Tablets.  

Tisha B'Av also was supposed to be the day that the Jews resolved to enter the Land of Israel and conquer it. The Sin of the Spies in slandering the Holy Land, which took place on Tisha B'Av, prevented the Jewish People from entering the Land of Israel.
When Moshiach comes, we will have rectified this Sin of the Spies too.

Thus, Tisha B'Av will assume its original intention of intense national joy of appreciating and cherishing the Holy Land of Israel!