...the North Carolina Collection at the University of North Carolina at Chapel Hill to plan our first ever edit-a-thon. Our theme is African Americans in North Carolina,

...where is the best place on Wikipedia for me to promote this event?

There is no one place that serves as a central promotion spot. As with social media, you have to get into various places to promote your event.

Don't forget to add your event to this year's Wikipedia Loves Libraries list: https://en.wikipedia.org/wiki/Wikipedia:Wikipedia_Loves_Libraries/Planned_events

Presumably the people in the local Wikimedia chapter are apprised of the event and will come.
Make sure the library and UNC will promote the event (widely) - don't forget the campus newspaper
Social media: Twitter, Facebook, and the usual suspects
There must be local venues/outlets (newspapers? websites? community centers?) that cater to the African American community in Chapel Hill and environs -- use them.
I note the date is April 16 - might you want a tie-in to Lincoln?

In short, as with social media, there is no "one size fits all" and you have to reach out in many ways (often) to promote your event.

Good luck!