From Walt Simonson’s Facebook page:
“Actually, the logo on the poster said ‘Marvel Mutant Massacre Map’. Somebody at Marvel felt that ‘Merry Marvel Mutant Massacre Map’ was in poor taste. So it was changed. But that was the original title. 😉
“The poster outlined the reading order for what I think of as the first true crossover in the modern sense in mainstream comics. There had been occasional character crossovers before this (Iron Man and Sub-mariner come to mind for those of us who go back far enough) and there was an Olympic-themed comic with a lot of Marvel characters in it around 1980. And Thor featured a winter invitational during the Casket of Ancient Winters storyline that a few other titles participated in. So the general idea was floating around in the ether. But AFAIK, this was the first time a series of books at either Marvel or DC were linked in quite this way.
“The collaboration really grew organically out Chris Claremont and Weezie. The story of how that happened, while not THAT long, is longer than I want to post here. But they began collaborating on what became this crossover based on a story idea of Chris’s for X-Men. I got into the act because of Weezie and before we knew it, there were five different titles involved in an overarching crossover. And it really happened because the three of us were involved in these five titles. We did try to write the individual titles in a way that gave readers who were following individual titles a coherent story in each title while at the same time creating a larger story arc throughout all the books.
“At some point, it was decided that a chart showing the various threads and their relationships was needed. Well, both needed and promotional, I think. 🙂 So I drew it up and added a row of figures across the bottom with a separate drawing. I don’t remember now if I did the actual paste-ups of the logos. At the very least, I would have indicated where every title went in which box.
“The entire assemblage was released to comic shops as a two-color poster.
“We all had a lot of fun, and the crossover was successful enough commercially that we found ourselves facing an editorially-mandated crossover the following year. And in general in comics, pretty much every year since.
“Nothing succeeds like success.”