For those who don't know, this light novel went live in Japan when season four began its home video release. My Hero Academia is releasing six volumes of season four to fans in Japan, and each of those bundles come with the novel which continues the Cultural Festival. And thanks to translator Aitaikicochi, an English summary of the story has gone live.
According to Aitaikimochi, the light novel follows the events for the wrap up party after the Cultural Festival finishes. Class 1-A has plenty to do with the event as its stage show lit up attendees and is described as being a huge success. That is why Aizawa even agrees to the party, so the light novel follows any preparation going into the wrap party along with the festivities themselves.