The cancellation protections are mostly a function of Florida law (assuming you are buying either VB or something at WDW), and that includes the mechanisms of notification. The date that matters is the date on which it is postmarked, not the date on which it is recieved.
The general advice from TUG is: send it certified mail (so you have proof of mailing). Make sure every purchaser signs the rescission letter. Include a *copy* of the signature page of the original sales agreement. Follow the instructions in the contract to the letter. Do not improvise.