Step-by-step explanation:
I would say may 6th because Sam didn't know Ralph was going to revoke until May 5th and that only makes it half final because Ralph wouldn't know in anyway except through a letter that Sam has received his letter and agreed or disagreed.
May 6th is when he gets the confirmation. So both people know on May 6th.
If this is too confusing ( like it was for me I had to read it 6 times ) then think about it this way. if you make a deal with a fisherman to buy fish on Wednesday and you send him a lettering sunday that arrives a day later, the fisherman won't know until a day later (Monday) and on that day he receives it you don't know if he got it. That's why it's half official. when he send a letter that arrives the day after he got your letter (tuesday) then you know that he understood you won't make it on Wednesday making it fully official.
does this make sense? if so hope it helps.