This extraordinary urban myth was based on verse 58 of his first Century, or book of prophecies. By whom, I have been unable to trace.
None of the established writers seems to have been responsible for this disgraceful canard. Which is surprising, since it is well within the capabilities of some of them.
In my verse translation, the verse reads:
The belly sliced, it shall be
And with four arms: four
whole years it shall live:
When Aquileia's festival is
Turin, Fossano follow
That's what it says. Honest. Simply the birth by Caesarean section of a pair of Siamese twins, and a bit of Italian politics. Nothing about World War Three, nothing about the end of the world, nothing about America, nothing about July, let alone 4 July.
What seems to have happened is that some Gallically challenged paranoiac, spotting the original French word "Alquilloye" in line 3, jumped to the conclusion that this was really the Latin word aquila (eagle). And since there is, of course, only one country in the world that has the eagle as an emblem (a wonderful piece of virtual geography, this!), the "day the eagle celebrates its festival" could only be 4 July.
Beware, though! Nostradamus made a real prediction for this very month - or rather, for 14 July to 13 August. Verse X.72 predicts, in effect, that a new Emperor Charles V (Schroder?) will restore to power a new Francois I of France (Prodi?).
Which, of course, has not stopped further Gallically challenged paranoiacs turning him into a virtual King of Terror from the sky.