For a cruise holiday, build your flight reservation around the ship's embarkation and disembarkation dates—not simply around the first and last dates of your vacation. If you are flying to Barcelona for a Mediterranean sailing, the sensible air itinerary often arrives at least a day before the ship departs and leaves after the cruise returns. That creates a travel plan with room for the port transfer, an overnight stay and the actual sailing schedule.
Consider a hypothetical seven-night cruise leaving Barcelona on September 6 and returning on September 13. They live in Istanbul and want to spend one night in Barcelona before boarding. Their proposed flight plan is Istanbul to Barcelona on September 5, then Barcelona to Istanbul on September 14. The extra day after the cruise is deliberate: they would rather have a relaxed final night than race from the ship to the airport.