In 7 BC, Herod the Great executed two of his sons for treason – Alexander and Aristobulus. At least, it happened in 7 BC according to major scholars on the subject as far back as 1885. [For the record, I agree, but there's more going on here. Keep reading.] Some of them admit the date is an approximation, which is technically correct. Daniel R. Schwartz went further, stating with more precise vagueness that it could have been 8 or 7 BC, and pointing out, “the dates von Gutschmid offered as approximations have all too often been accepted as canonical.” (Agrippa I, p.39.n.4; p.206,n.15) Fair enough. So let's do better.
The passages in Josephus are not his most helpful, for chronological precision. A good starting point is the accession of Aretas IV, King of Nabatea, which happened in the winter of 9/8 BC according to various sources, including [if memory serves] coinage. That puts the visits of Herod’s ambassadors, Nicolas and Olympus, squarely in 8 BC, but more precision is needed. Estimating their separate travel itineraries leaves a range of possibilities, and it is admittedly (if barely) possible that the double execution might conceivably occur as early as October of 8 BC.
Schwartz, then, has good grounds for his rigorous skepticism (at least in this case). On the other hand, none of our evidence requires an expedient timetable and so 7 BC remains - statistically alone - far more likely than 8. But statistics isn't argument. As far as I can tell, one early justification for the consensus position was originally that Saturninus disappears from Josephus’ narrative shortly after the double execution [and Saturninus leaves mid-6] so Herod's sons were more likely to die in 7 than 8. This doesn’t seem like a very strong case, and I’d like to see better.
But of course, I'm not merely trying to be thorough. The question at hand depends on the travel itineraries of two embassies from Herod to Rome, which also determine whether Augustus reinstated Herod's friendship in early 8 or late 8, and give us a better picture of just how long Herod the Great was officially a "subject" of the Emperor.
To be honest, I'd never looked at the logistics of 8 BC quite this closely until I found Schwartz's footnote. So hooray for footnotes!