Actually, the OFFICIAL date of entry into the Registery is when handover occurs from the Builder to the Operator.
The naming ceremony is just a show really.
For example, I seem to remember that AidaStella was handed over from Meyer to Aida on 10/03/2013. The actually Naming ceremony was on 16/03/2013
Some ships do several official sailings with passengers before the Naming Ceremony because of commercial reasons ( like press promotion events, homeport, etc...)