A fiscal year is a company's business year basically. They calculate their finances and it shows how well they are doing, and in what areas, etc...
For Nintendo, they finish their fiscal year March 31st, so they get their finance information on March 31st; it's probably so they have all the Christmas sales and post-holiday sales accounted for. Then they can budget and create marketing strategies etc, starting 2nd quarter of 2011.
Now they said Skyward Sword would come out early 2011. So Gamestop is taking that into account and just throwing the release date at the end of the first quarter of 2011. 12 months a year, split into 4, is 3 months a quarter. April 1st is the end of the first quarter. It's just a place holder on what we already know.
Anyways, enough ranting. Basically they are aiming for early 2011, so it could very well be January 15, or it could be delayed till June. Who knows? It's Nintendo lol. But I really hope it's released January!