This day is the beginning of the nine days, from the beginning of the month of Av until the 9th of Av.
In addition, this day marks the yortzait, the anniversary of the death of Aharon HaCohen, the brother of Moshe.
The Torah tells us that Aaron loved peace, pursued peace, loved creatures, and brought them closer to the Torah.
Such was the affection the Jewish people had for him that they mourned his death for thirty days, "all the children of Israel," which includes the women.
When Moshe died, the Torah recounts that only the men mourned him for thirty days.
From this we see the importance of the peace that Aaron pursued, to the point that our sages say that the only reason the Torah was given is to make peace in the world.