Generally the guideline for booking shore excursions is up to 10 days before departure. Some cruise lines allow only up to 2 weeks before. After that time, you have to wait until after you have boarded the ship to make any requests or changes. If you do wait until boarding, make sure to go to the shore excursion desk onboard as soon as they are open (which will probably be sometime during the first evening onboard, usually after the ship has left port and is at sea - listen for announcements and check the daily onboard activities newsletter for those hours.) Have a great time!