This is according to a report from Famitsu (via Attack of the Fanboy) who revealed the date. However it was not mentioned as to whether or not this date applies to other markets, but given that Metal Gear Solid V is an international release, there is a good chance that the DLC’s release for other markets will also be on the 15th of March, or at least roughly around the same time.
For those unfamiliar with the DLC, this basically takes the Quiet character from the game and makes her a playable one in the Online mode. Given that the character was non-playable in The Phantom Pain, we’re sure many gamers are curious to see how she will perform as a playable character. It will also come bundled with three new maps for players to check out.
In any case the 15th of March is only a couple of weeks away, so keep an eye out for the DLC if you’re looking for newer content to play with.