The SBPLI Regional date is set for the 5th week, but the venue contract is just a handshake right now. While the date is public knowledge, until the contract is signed it won't be officially published.
FIRST headquarters has to agree with the proposed date so as to insure a field is available, but venue availability is the primary driver. Those venues that have several open weekends can accommodate being shifted for whatever reasons you choose as Jon mentioned, e.g., to spread out the playing field usage or to avoid over saturation in an area for teams and volunteers on a single weekend.
The 6 months official FIRST notice we get (September - March) is probably sufficent at least for the local team attendees. I get less notice about our school events calendar...
I sympathize with locking in airfares from Hawaii
. We always signup for Championships during the open registion period, because we don't want to be buying tickets if we qualify after week 5.