"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.
I would think the よ would be more for emphasising the "is" in the English translation, as in "It is your birthday today, right?" Meaning person A is under the impression it's Person B's B-day (pun intended), but is doubtful and wants to confirm, hence the ね. I think ね alone would fit your explanation better of it being more rhetorical. I could still be wrong though, it's just my interpretation
THE YO makes it more of a string statement. Like "I know today is your birthday, right?" Making it more of a statement than a question. Whereas omitting the yo would make it absolutely a question which infers you forgot which someone might take offensively whereas they would not take the ither form offensively. Little verbal tactics...