Walters is retiring that day after a storied television reporting career that has spanned five decades.
De Blasio made the announcement Monday when he appeared on "The View," the daytime talk show that Walters co-hosts.
He gave her an official city proclamation.
Walters and de Blasio's predecessor, Michael Bloomberg, had a frequently flirtatious on-air relationship.
De Blasio and his wife, Chirlane McCray, both appeared on ABC's long-running talk show. The show's hosts mostly wanted to ask about the couple's relationship and family, and the conversation largely avoided politics.
De Blasio, who is white, and McCray, who is black, spoke about the challenges of being an interracial couple.