After clicking on the link above, I read the part that applies to our cruise. I don't understand most of the information about commodities trading or barrel prices, etc. but I do understand the part where it says that if prices stay below $70 a barrel, then the FIRST cruise that would get the credit will be the ones sailing on December 18. I should have already known this. Now it's very simple to understand - because we leave on the 15th. So, naturally the earliest would be the 18th.
I guess I'll just have to hurry up and book another cruise.