I had some pretty strong reactions to my first 2 Pfizer vaccines last January (including a massive, itchy, rash all over my trunk that was reported to VAERS website) but nothing at all with the booster. In fact, my flu shot was given several days before the covid booster and that hurt longer than the covid booster.
I planned it for 15 days prior to our cruise but I would do 6 days prior. Probably not any less than 5 days, just in case. Also if you're going to get it 6 days prior, I would pack and do as much as possible before getting the booster. Then, if you do feel a little crappy, you won't have a lot of preparing to do while you're not feeling well just prior to leaving.
As others said, it can't cause a false positive.