I posted a tweet in the OP, but here's a video from Gamexplain regarding the matter with date and time for NA.
Ha I had already ninja edited my post! But yeah I did see the tweet but since that Stealth guy is a leaker I assumed that was rumor, but nope Nintendo did indeed officially announce the dates, atleast for the Japanese version. I'm assuming there's also going be one for Europe and another for America, just like with the Splatfest demo. That'll give me ample opportunitys and time to decide if this is the first Mario Tennis game I play since the N64 version!