I've stated this before, and I'll state it again.
Assuming McCain is still alive (and since he's made no public appearances in some time, we can only assume that), he's holding his resignation back until after May (not sure exactly which day, but it's a date in May).
That's because AFTER that date has passed, the governor of Arizona will APPOINT a replacement for him (rather than call for a special election), and that replacement will serve until the 2020 elections roll around.
I proffer that some kind of deal has already been cut with the governor (either by McCain himself or perhaps by his wife and daughter), the crux of it being that Cindy McCain will be appointed to succeed her husband.
The first time I posted that, someone replied that Cindy McCain isn't well-liked in Arizona and that won't happen because of such public sentiment.
It DOESN'T MATTER whether or not Mrs. McCain is "liked" or not by the voters.
The whole point of the deal is that the voters will have no say in a gubernatorial appointment. Cindy McCain will get the seat anyway.