Quick run-down, at a recent U2 show, someone (Bono or Edge or someone in U2) mentioned that they were talking with Green Day and Metallica and that they wanted to play together (or something like that). Then an article came out talking about the "G3" meeting between the 3 bands.
And we can assume (maybe incorrectly) that's what Billie Joe was talking about in his tweet
A festival? With GD? It's my nail and your coffin pal..
Now, anyone feel free to correct me or add more detail as this is about the extent of my knowledge of this.