It was announced by the official One-Punch Man website that there will be a special anime episode for the series airing on April 2, which is one week before the official debut of the show’s second season. The second season will be streamed on Hulu, according to Viz Media who holds the license to publish the manga here in North America. 

The website states that extra episode is a broadcast premiere commemorative special. As for what that means, remains a mystery. It could be just a silly OVA episode, it could be a recap episode of the first season. It could, literally, be anything. No details are not really any surprise as this is the norm when it comes to Japan and anime announcement.  What we do know; however, is that a Key Visual was also released for the show, which you can check out below:

Source: AnimeNewsNetwork