Pittsburgh Post-Gazette:

Fahlman posted the emoticon in a message to an online electronic bulletin board at 11:44 a.m. on Sept. 19, 1982, during a discussion about the limits of online humor and how to denote comments meant to be taken lightly.
“I propose the following character sequence for joke markers: :-),” wrote Fahlman. “Read it sideways.”

Several years earlier, in 1979, the “tongue-in-cheek” icon -) failed to make a similar splash.