Ron Shorts

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Email system is Office 365, running on Windows 10 with click to run outlook 2016 with 20k+ users., have share point online as well.<br />
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From what I've seen would be to create a custom holiday file (.hol), and run it as &nbsp;gpo startup script (this doesn't cover any outlook online however, just local client). &nbsp;Didn't see any native group policies for this, or office 365 method in exchange you could manage this though.
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If you have Office 365 at that scale, I would invoke the Office 365 support to ask their recommended solution. The environments I typically work with is Outlook local and online, and they are up to 50 users per client. So a share once solution is kind of easy for my scenario, but not at 20k+ I imagine. Please keep us updated when/if they give you a solution as it may be good knowledge to pass along.
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What is the best method to add holidays to everyone's calendars company wide in Outlook? &nbsp;<br />
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Running Outlook 2016 click to run in exchange online. &nbsp;Also need to put in custom holidays?<br />
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Thanks experts!
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What is the best method to add holidays to everyone's calendars company wide in Outlook?  

Running Outlook 2016 click to run in exchange online.  Also need to put in custom holidays?

Thanks experts!

Adding Holidays in outlook/exchange to everyone's calendar

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.

Exchange

Microsoft Outlook is a personal information manager from Microsoft, available as a part of the Microsoft Office suite. Although often used mainly as an email application, it also includes a calendar, task manager, contact manager, note-taker, journal, and web browser.