My high school computer science teacher (OK, he was just and interested math teacher) got an Atari 800 about a year before I bought mine. In fact, see its graphics abilities was what influenced me greatly. His classroom had several PETs. He just could not figure out a way to to get the computer to fit into his, er, program. I bought my 800 in December 1980. That means he bought (and returned) his 800 in the November/December 1979 time frame. It was definitely prior to Christmas break 1979/1980. This was in the middle of nowhere, in Western NY. (If ever there is a shining center to galaxy, we live in the city that is farthest from ....)
My 800 has a hand written serial number which suggested an early November 1979 manufacture date, if I read it correctly.