It started with "a new line of miniatures rules" and became a global phenomenon.
Enlarge / The three rulebooks fo "fantastic medieval wargames" that started it all, released at some point in late January 1974, as seen in Dungeons & Dragons Art & Arcana: A Visual History. (credit: Wizards of the Coast/Ten Speed Press)
"We have just fromed [sic] Tactical Studies Rules, and we wish to let the wargaming community know that a new line of miniature rules is available."
With this letter, written by Gary Gygax to wargaming zine publisher Jim Lurvey, one of the founders of what would become TSR announced that a January 1974 release for Dungeons & Dragons was forthcoming. This, plus other evidence compiled by Jon Peterson (as pointed out by the Grognardia blog), points to the last Sunday of January 1974 as the best date for the "anniversary" of D&D. The first sale was in "late January 1974," Gygax later wrote, and on the last Sunday of January 1974, Gygax invited potential customers to drop by his house in the afternoon to try it out.
You could argue whether a final draft, printing, announcement, sale, or first session counts as the true "birth" of D&D, but we have to go with something, and Peterson's reasoning seems fairly sound. Gygax's memory, and a documented session at his own house, are a good point to pin down for when we celebrate this thing that has shaped a seemingly infinite number of other things.
