Piazza della Riforma is the Main square of Lugano, right in the middle of the City center (highway exit Lugano North or Lugano South).
Piazzale alla Valle is in the center of Mendrisio (highway exit: Mendrisio, about 15 min. drive, from Lugano City Center. Direction: south).
Don't "dare" to park in the center of Lugano during the evenings of the concerts. Use the parking places "Cornaredo" or "Centro Esposizioni".