Hi @Tom Sweeney
Currently you can add holiday calendars for different countries in your Outlook, but you cannot sync them to the channel calendar.
To get this feature more quickly, you can send a feedback directly to Microsoft Feedback to add it in future updates. Many features of our current products are designed and upgraded based on customers’ feedback. With requirements like this increase, the problem may well be released in the future.
As an alternative, ask users within the channel to add the calendar in Outlook and view it in Outlook.
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