I'm new to this whole pre-Pokemon hype thing (I don't like following a game before its release, and then not following it after its Japanese release to avoid spoilers while waiting for a localization), so I don't really know how this works. People act like they know exactly when it's coming because, I guess, they've done it every month since the dawn of Pokemon.
Socran's Razor: "Never attribute to massive stupidity by someone else, what could be explained by a tiny bit of stupidity on your own part."
Usually around the 10th-15th of each month. Serebii and Pokebeach are good sources for info. I personally prefer Serebii, but Pokebeach sometimes posts rumors that may or may not be true, while Serebii is always sure to double-check their information first.