Hi @Chris Church ,
Welcome to our forum!
Looks like your issue more related to macro, please kindly understand that the Outlook tag here we mainly focus on general issues about build-in feature in Outlook desktop client. Based on my test, i found that after you overlay several calendars in Outlook and then restart Outlook, the selection will be preserved. In this way, you can keep the calendar you want to overlay without using macros.
If you insist on macro, based on my research, i found an article which mentioned how to keep several calendars overlaid whenever you shift to the calendar area, please refer to:
How to Auto Overlay Specific Calendars When Switching to Outlook Calendar Area
(Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)
If it doesn't work, in order to better solve your problem, we will add "Office Development" tag.
Thanks for your understanding and hope your issue would be resolved soon.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.