For festivals: Many NGOs have a summer break, which usually coincides with the respective school vacations of the state or country. During this time, there are often only a few volunteers available, and the NGOs are sparsely staffed. If possible, consider such times during your preparation phase or festival days.
However, some youth NGOs are the exact opposite; they are most active during the summer, when there is no school and they can work with young volunteers. They may already have numerous other activities planned, and their limited resources might already be stretched out.
If your festival takes place directly after the summer break or at the end of the summer break, make sure you have organized everything before the break. This will save you a lot of stress close to the event.
For NGOs: If the festival takes place shortly after a holiday break, make sure you have organized the most important things such as contracts, material, volunteers, and accommodation before the break.
School holidays in the United Kingdom, Spain and Italy:
https://www.dein-dlrp.de/en/trip-planning/best-time-to-travel/european-school-holidays.html