damn how much time passed between chapter 18 and 19
did a whole summer pass between the chapters, they're preparing for the same event as they were in the last chapter though right
Japanese schools have three semesters, separated by vacations.
Summer vacation comes after 1st semester, and usually covers the 40-odd days from July 20 to August 31.
Chapter 17 happened on the last day before summer vacation, and then they come to school on another day in summer vacation for preparing the festival in chapter 18.
I think the time span from chapter 18 to 19 is no longer than one month? Since they already have a checklist for their after party program in chapter 18, it looks like it was not their first meeting in summer vacation.