On what date was Yahoo's agreement to acquire Tumblr announced?

Yahoo’s agreement to acquire Tumblr was announced on May 20, 2013.

Yahoo and Tumblr announced a definitive agreement under which Yahoo would acquire Tumblr for approximately $1.1 billion, primarily in cash. The announcement was a major move for Yahoo chief executive Marissa Mayer, who presented Tumblr as a fast-growing community that could expand Yahoo’s reach among younger internet users and strengthen its mobile and media businesses.

The deal was structured to preserve Tumblr’s identity as a separate business. Founder David Karp remained Tumblr’s chief executive, and Yahoo said the service’s product, team, and culture would continue to operate independently. Those assurances mattered because many Tumblr users worried that Yahoo might change the site or weaken its distinctive community.

May 20 is the public announcement date, not the closing date. The transaction was later completed in June 2013, which explains the common mix-up with June 20. April 1 is also a plausible-looking distractor because Yahoo’s announcement was widely treated with skepticism, but it was not the date of the agreement.