I can't really tell you a source; 'cause it's something like "people say", so I don't know... perhaps it's not so right after all!
Ok, but I haven't dream with Green Day so much. There were very few, and I can't remember most of them. My dreams are always like blurry...
Anyway, for example; once I dreamt I was going to a concert but I met Billie Joe outside the stadium. I thought "this is my opportunity to ask him an autograph!" so I approach him but he was angry at something and that scared me a little ( ). He was angry filling some papers and all of the sudden his pen stops working (which I think made him more pissed) so I told him I could lend him mine but with the only condition if he could please give me an autograph. He did. But he wrote A LOT. And never said a word. When I look at the piece of paper I can't understand a thing of his words and drawings, but I thought it was cool anyway! Oh, and while I was trying to read he just dissppeared.
Weird and simple, huh?