Exchange 2013: Read specific dates (with categorie vacation) in foreign mailboxes?

Hi there,

we would like to output a list of all holidays in a list.
We have to search through all existing mailboxes and filter the entries with the category "vacation".

Is there a way using PowerShell or otherwise a list of all vacation days to create?

In any calendar read access is possible

Someone an idea or code example?

Thanks in advance,
regards,

Coyo