On further reflection, I don't think we have a flag day, at least not
what I understand as a flag day. A flag day is a day on which
everyone has to simultaneously make some change, otherwise things stop
working. I don't think we have one of those.
Assumption: Even after flag day, it's still legitimate to reject on
PRD, rather than being required to use the envelope sender for those
SMTP transactions that don't specify RFROM. Meng implied this, by
saying that tools like SpamAssassin will continue to get the PRD from
the headers even after flag day.
On flag day, rejecting based on envelope sender for such transactions
becomes permitted, possibly even recommended, but not mandatory. So
old SPF validators remain conformant. Someone checking SPF records
doesn't _have_ to do anything on flag day.
The flag day isn't really a flag day at all. It's a deadline, by
which forwarders need to implement _either_ RFROM _or_ SRS.
Slightly more obscurely, it's also the deadline by which any SPF
publisher that for some reason needs to avoid checks based on the
envelope sender _must_ implement RFROM.
But nothing special has to happen on flag day. With luck, everything
will have happened in good time before the flag day.
So it's not actually a flag day, it's a deadline.
-roy