There is so much information about this available online it is a wonder you think you have the basis to argue about it.
All Saints Day. November 1st, is a national holiday in Italy and a Holy Day of Obligation in the Catholic Church. Everything is closed in Italy and everybody who is Catholic has to go to church. All Souls Day continues the religious feast, but it is not an official national holiday or a Holy Day of Obligation.
Traditionally, All Saints Day is a holiday focusing on gifts for children, celebrations of food, and on both days, visits to cemeteries to honor dead ancestors. I don't find any mention of a tradition of costume-wearing for All Saints Day or all All Souls Day.
Halloween is catching on in Italy but is not traditional. The students in Perugia probably celebrate it in part because there are so many foreign students there. Everyone in the cottage went to Halloween parties and dressed in costumes on Halloween night, hence the pictures of Meredith in her vampire costume.
As in the United States, having a holiday the next day means parties the night before. All Saints Day is a holiday, so everyone parties on Halloween. All Souls Day is a feast day, but not everything is traditionally closed, although in 2007, it was more a holiday than usual because it made a long weekend. The evening of All Saints Day (when the murder was committed) was a party night for some -- Meredith went to her friends', Amanda went to Raffaele's, Rudy went to a disco -- but it was not a night for costume parties.
Curatolo was referring to Halloween.
If you want to argue that some people may have worn costumes on All Saints Day, you will only be able to get as far as claiming they were dressed up as saints -- not as witches or with masks, etc.