In the 1830s "OK" was part of local Boston slang and was propelled to national prominence when it first appeared in print in the Boston Morning Post. That happened on March 23, 1839. OK gained in popularity when it was picked up by politicians, thus securing its seat in history.
A bit of sleuthing by the late Allen Walker Read, a professor at Columbia University, fleshed out all the incorrect attributions to the origins of OK and Read's efforts led to uncovering OK's true origins and it's March 23 birthday.
Now, who is going to tell us when OK gained the correct spelling of “Okay?”
Read more on the History Channel's page: history.com/this-day-in-history/ok-enters-national-vernacular