Every school needs to set their calendar at the start of the year. This can be done via Admin > School Options > Calendar Setup.
You can then adjust the start and end dates for the term - these should include any INSET days etc.
The next step is to click on the relevant dates to change the code from green (school open) to yellow (INSET day etc.) or red (school closed).
NB: This needs to be done so that your attendance data is accurate.
If you are closed unexpectedly then an admin user will need to remember to do this for any dates that apply.