One is currently only able to allocate a timetable to whole weeks rather than individual days. This means when you have half terms etc which run from mid-week or to mid-week you need to create 6 half terms rather than 3 terms. If you have a single day when the schools is closed to students, it is not possible to "unallocate" that day (eg Black Friday has become a fairly common day to close schools to students).
