Hello,
Currently, only by applying workarounds or by designing a module for this specific purpose. We have plans to create such module, but it's pretty low on the priority list.
The workaround is not very efficient, I must admit. It requires to create events (like meetings) with all interested employees assigned. If the holidays are occurring on the same day of the year, you can use the recurring meetings for that purpose.
And as we do not provide a mechanism for importing meetings/tasks, etc. I'm afraid it has to be entered manually or imported through custom script.
Kind regards,
Arek