The way the dates are chosen for the 12 competitions in 12 different cities and states is this. We have a 4 week (or 5 weeks with Easter) stretch, during which the competitions can take place. It is completely up to the local organizers to select the weekend for their competition. Typically, organizers avoid trying to judge on Easter weekend, and during a known spring break time, plus they also try to balance the date to not conflict with surrounding/regional competitions, etc.
In any given year, it is entirely possible that all 12 competition will occur on the same weekend. If this is the organizers' choices, then that is the way the competition will happen. Keep in mind that not all 12 competition winners lists will be posted simultaneously since the results must be verified by me and John Moorhead before each winners list is posted.
BTW, the competitions are all registered with the BJCP in December, so the dates of the different locations are available on the BJCP calendar well ahead of the competition registration process.