Colleagues in my IT company are generally quite intelligent and capable of using a shared calendar to inform people of their movements (including holidays).
However, a small number bizarrely choose announce their holiday plans as a meeting invitation to all and sundry.
For example,
"Michael Brown has invited you to 'Vacation in Yosemite' from Monday 15 January 2018 to Friday 19 January 2018."
Without exception, I always accept/decline the invitation and always reply directly to the author:-
'That really is most kind and I will be delighted to join you in Las Vegas next week.'
or
'Thanks awfully for the invitation but due to the short notice and problems securing a visa to enter Russia, I'm afraid I won't be able to join you in Moscow.'
Sadly, I am met by a deafening silence but occasionally someone replies
'Eh ? WTF ?! But I never invited you to Vegas...'