The day of acknowledgment for the buns....aka "butt". The acknowledgment can be as simple as a pat on the butt,
all the way to full blown anal sex.
As long as some kind of bun acknowledgment takes place on a Sunday, that Sunday becomes Sunday bunday.
Once the Sunday bunday tradition has started, it is expected to continue every Sunday with some sort of acknowledgment, whether it be just a verbal announcement of it being Sunday bunday (preferably when you wake up
in the morning), all the way to full blown anal sex (preferably when you wake up
in the morning).
Sunday bunday is the day that directly follows Scaterday.