Once you are on a polyphasic schedule, the concept of a "day" becomes more
difficult. This is one reason why puredoxyk gave names to each night of the
week as well.
Puredoxyk, if you're listening here, what were the names that you used for the
nights of the week?
If you want to stick to calendar days, I'd think that it would make the most
sense to designate "day 1" as the first calendar day without any monophasic
sleep, but as others have noted, you should make your own rules here.
Cheers,
--Ian