If you've looked at a radar today, you will see that a wall of rain is on its way to the NYC area and because of that, the Yankees have postponed tonight's game. The Yankees and Pirates will play a single admission doubleheader Sunday.
The deal is if you had tickets for tonight, you can exchange them for tickets to a game at a later date. If you have tickets for Sunday, you get to attend the doubleheader.
UPDATE 1:15 P.M.: Yankees PR says that if you have tickets for tonight you can exchange them for Sunday tickets if you'd like and that's subject to availability, of course.