Taco Tuesday as a concept started much earlier than commonly thought, with businesses advertising tacos on Tuesdays as far back as 1933. Specifically, the White Star Cafeteria in El Paso, Texas, ran a weeklong campaign in October 1933 to promote Mexican tacos on Tuesdays. The phrase "Taco Tuesday" itself was first documented in print in 1973. The widespread promotion of Taco Tuesday in Mexican and Tex-Mex restaurants notably grew in the 1980s to boost slow midweek sales.
In summary, Taco Tuesday started as a special deal tradition in 1933, with the specific phrase becoming popular and widely used from the 1970s onward, and gaining mainstream restaurant adoption in the 1980s.