"Today is your birthday, right?"
よ and ね are not quite opposites in meaning. よ adds emphasis, and depending on what is being said that emphasis can be argumentative, agreeable, or simply, well, emphasis. In the first case it would seem somewhat opposite of ね. As you know, ね is kinda like "right?" or "isn't it?" in English (though that is usually an awkward translation).
The Japanese often use よね when talking--often when talking with energy or emotion--about something that is taken by the group to be fact or otherwise granted. It's sort of a rhetorical question sentence ending. In this case, Person A saying "Today is your birthday, right?" isn't really asking if it's Person B's birthday, but is simply using the question rhetorically to bring up the subject of Person B's birthday, since both Person A and Person B know it's Person B's birthday. For this sentence one can use ね by itself, but it lacks the emotion that よ gives.