The game's producer, Shigesato Itoi, has revealed the Japanese release date via his official website.
Mother 3 (the series is known as Earthbound outside of Japan) hasn't had the smoothest of histories. Originally planned for the Nintendo 64, the game was cancelled after six years of development. The sequel was given a second chance in 2003 when Nintendo revealed that it would be released on the GBA, albeit in a very different form to how it was originally imagined.
There's no word on a US or European release date, but gamers in Japan will be able to get their hands on the game from April 20.