Eldar Murtazin, a person known for getting it right when it comes to Nokia leaks, has tweeted, "Save the date for a big announcement - March 14." When it was suggested that this could be the Samsung Galaxy S4, he responded with a "Yes!" and pinpointed the announcement location as New York.
Of course, this doesn't mean that the launch is confirmed. Far from it, actually. Murtazin has previously been spot on when it comes to Nokia rumors, though his oft-suggested buyout by Microsoft has yet to happen. However, other rumors have placed a Galaxy S4 unveiling sometime mid-March, so we're starting to think it's going to happen.
Full details aren't known yet, but I'd keep mid-March open if you're planning on attending or watching the announcement.