The Festival

This is certainly not the first time that Yokai Academy has organized an event of this magnitude. But when music day arrived, they went so far as to organize a 3-day music festival.
For these drunken festival-goers, there will be plenty of sex and questionable hygiene but it will be all the more delectable to ignite their souls with divine music. Rias and her group are obviously invited.
The opportunity is too good to pass up. From then on, Issei and his darlings will be able to reveal parts of themselves that they have until now kept hidden.