Scientists look at the historical peaks from the last years, then they work back from that targeted date by adding in the many months needed to do trials, manufacturing, etc. Good thing is that, with mRNA technology, that time window is actually much better that it used to be for the older tech we’ve used for flu viruses in the past.
It’s never going to be perfect, but as long as you had last year’s booster, as the article mentioned, you’re much less likely to get seriously ill.
Predicting when next year’s peaks will drop is like predicting next years rainfall. You can get close, but it’s never going to be perfect. (Not the greatest analogy, since rain doesn’t mutate, and umbrellas don’t require months of human trials. Ohh well)
These are smart people working on this stuff. If they could have had an updated vaccine ready to go for any unexpected peak, they would.