By Chapter 17 we now see that every community member's time is scheduled fairly rigidly. There does not appear to be much individual choice about how to spend one's time. While children do have play time and relatively few obligations, adults are required to go to work and perform their expected duties. Everyone must be productive in service to the community. Days off are, of course, scheduled as well.
The significance of the "unscheduled holiday" is two-fold: first, just like in our own world when we have an unexpected day off from obligations (like a snow day, for example), we get really excited because we can do something of our own choosing and usually something fun. Second, the wording, "unscheduled holiday," indicates something serious may have happened to warrant such an unexpected day off, which holds even more meaning in a society that strictly manages their time.