Just make the skipper / team check of which days they will be racing at registration, problem solved. This could be done during online pre-registration or during any registration period. Anything that is 'open to interpretation' can get tricky / political.

As for what days, I see the point of having either the first or the last as 'optional', that is up to the PRO to decide.

I'll use my boat this year as an example, my skipper was suffering from food poisoning (or something with similar symptoms) and was unable to walk let alone sail on the Friday. He came to the beach to see if he was up to it, and after we registered we decided it was unsafe to sail (he nearly passed out stepping the mast). Our Friday scores were then given our average over the weekend.

We definitely benefited from the 'Friday is optional rule', but the reason we opted out of Friday was done without any 'sneaky' intent, simply due to safety concerns (trust me, flying down from Canada for a three day regatta, you want to race all days). That said, if we had had a bad race, or any breakage, the average score would easily have been way higher and not worked in our favor, so it is still a risk to miss a day (tip once, get a last place, ouch, up goes the average).

I guess my point is just make it clear on the registration sheet if someone is sailing or not on any decided optional days, and stick to it. Asking people why they are not sailing, or defining 'here' or 'not here' leeds to a lot of gray area.

Just my two cents, had a great time, was glad it heated up a bit and more red cups, more beer and offset marks ;-)