OTOH, last-minute is a great time to book unsold cabins sold at discount prices. I've only booked 2 cruises well outside the final payment window.

As far as South America ("antarctica") goes, October is usually the start and April is the end. Similarly, Alaska cruises usually run from April to October.

The cruising days are usually a strict necessity for getting between ports as most cruisers want stopsl, but sometimes there is no (practical) place to stop hence multiple sea days (usually trans-oceanic repositioning cruises, or you're in the middle of the ocean such as getting to/from Easter Island). My last cruise left Honolulu and didn't stop anywhere until it got to the destination port in the Sea of Japan: Cruiseco claimed it was 13 nts but they forgot the IDL (really) so it was really just 12. Still feel ripped off.