According to the Carnival site there will be 2 formal nights on a 12 day cruise. I believe the other nights are "resort casual". I don't think Carnival has any semi-formal nights, but I could be wrong .
Don't know about the masquerade ball, but someone else will probably be along to help you!
Have a wonderful cruise!
"A truly happy person is one who can enjoy the scenery on a detour."
Golden Princess 2009Carnival Conquest&
Grand Princess 2010Island Princess&
Grand Princess 2011Carnival Magic&
Carnival Dream 2012Island Princess&
Carnival Magic 2013Carnival Legend 2014&
Carnival Magic & Coral Princess 2015
Carnival Liberty 2016