No, this fest is not so important as you are thinking, there is only a blessing in the palace, not a mass , it lasts only 15minutes , there is not a torch light procession but a fire on the place of the palace and the paladienne are dancing around, the member or the members are attending on the balcony and it lasts only some minutes.
there are a mass in a Church in Monaco and dances on the place des Moulins on juni 24th but the members of the princely family were never attending this event
The Sainte devote fest in january 26 and 27 th and the procession of the good friday before the fest of easter are more important for the principauty than the fest of Saint Jean. This fest on the place of the palace was made in the years 70
Caroline and her family were at Paris, Albert was at Lausanne for the CIO and Charlene was in Greece for the christening of the baby of friends, she is the goldmother
Only the twins were in Monaco , but too Young to attend this fest