I use Sun Surveyor to pin the date and Photopills (which has a cool Augmented Reality mode) to determine when and where to look. I think Sun Surveyor is pretty much self explanatory, but Photopills will take you some time getting used to; there are tutorials for Photopills available on YouTube and I highly recommend watching those.
That leaves the weather. In landscape photography we usually aim for dramatic weather, but for Milky Way photography we obviously need clear skies. If you live in Europe, then check out modellzentrale.de.