1. Teachers do not schedule those in-service days, so it's not an "accommodation"; it's the thick bureaucrats that created this idea. And, ask any teacher, 90% of the time, it's a waste of a day. We'd much rather be in the classroom.
2. The teacher's union convention has nothing to do with the school calendar. And, the union is an entity that loosely represents the political opinions of its members.
Whoa!

How did my post generate that response?!
I was not trying to start a quasi-political argument. I was not commenting on the education system. I only suggested that week 40 might be in higher demand because some school districts in the western states may not be in session for at least part of that week. I see you live in California. I don't. I raised my children in Utah and every October they hold UEA convention. The children have no school on Thursday and Friday of that week and often only a half day on Wednesday. Whether you think that it is right or wrong is not the point of my response to the original question.
I was suggesting a possible answer to the OP's question on why week 40 in California has a higher demand. I was not commenting, nor soliciting comments, on the educational issues in California (or anywhere else).
To the OP--Joe, I checked UEA's convention dates and they don't correspond with week 40 this year so that is not the explanation--at least not for the Utah families that might be traveling to CA. Still don't know why it would be higher than week 39 or 41. My sister teaches in a year-round school and she is off-track that week (the kids would be out, too) but I don't think that would explain week 40 being in higher demand as 2/3 of the children would still be in school as well as all of the traditional year students. So, I don't think it is a public school issue affecting week 40.
Have you called the resort to see if they have an explanation?