Tu B’Av Established as Yomtov Because of Shemiras Einayim:
The Gemara states (Tainis 26B): “Rav Shimon ben Gamliel said: Yisroel did not have yomim tovim like the 15th of Av and Yom Kippurim. On this day, Jewish girls would go out with borrowed white clothing so as not to shame the ones who didn’t have...And what did they say, “Young man, lift up your eyes and see what you want to choose. Don’t set your sights on beauty. Set your sights on family...”
Sefer M’Zekeinim Esbonen quotes Rav Mordechai Chaim of Slonim zy”a as explaining the greatness of this day as follows:
From the fact that the girls had to tell the bochurim to lift up their eyes to look at them in order to choose one of them to marry, we can discern that they normally never lifted their eyes. It is worthwhile to establish a Yomtov to commemorate their incredible level of shemiras einayim.
