This calendar is to assist with showing your availability on the forum. When you are not available due to a holiday or busy period, then please add it here by using the “Insert date/time” option in the toolbar. When a @staff user is on holiday or absent, then a special flair is added to the username and in the tag when you mention the user. This is to help manage expectations from each other. This particular feature is only visible to @staff users.
An example is shown below. Notice the 
and the calendar icon in the user tag, which normally looks like @Kees.
When you provide your “Region” in your profile settings, then your national holidays are automatically added to the calendar.