Quite by chance I came across information on the Saga website about the timescales for booking excursions for Saga ocean cruises for the remainder of this year. I thought it might be helpful to post the link (for those people, like me, who were ignorant of the existence of this information!):
https://travel.saga.co.uk/cruises/ocean/the-saga-cruise-experience/explore-ashore/excursion-availability.aspx