@dew1later You will see many posts and conversations on the forum about the calendar. You can view these by searching ‘calendar’ using the spyglass search option. Bottom line is, because the calendar is not used consistently (and sometimes not at all) you are best to only consider the dates marked with a black line through them as the sitter being unavailable, as they are confirmed THS sits.
There is also a detailed explanation of the Sitter availability calendar that you may find helpful.