From Carnival’s website:
Except where destination requirements differ*, guests who are up to date with their vaccines may take their test within three days prior to sailing.
Up to date means a guest has received all recommended COVID-19 vaccinations, including any booster dose, when eligible.
Example: For a sailing departing on Saturday, guests may take their test as early as Wednesday and as late as Saturday morning, if guaranteed to receive results in time for check-in.
If a guest is fully vaccinated but not up to date with their vaccines, they must take an antigen pre-cruise COVID-19 test within two days prior to sailing, or a PCR test within three days prior to sailing.
If you are eligible for a booster and have not received one, you are not considered up to date.
Example: For a sailing departing on Saturday, guests may take their antigen test no earlier than Thursday and as late as Saturday morning, if guaranteed to receive results in time for check-in. A PCR test may be taken no earlier than Wednesday prior to sailing.
So, while the booster is not required, having the booster allows the traveler to take the Covid test one day earlier than those who are not boosted.