For the last couple of years the most constantly popular post by number of views on this blog has been Export calendar Items to a CSV file using EWS and Powershell closely followed by the contact exports scripts. It goes to show this is just a perennial issue that exists around Mail servers, I think the first VBS script I wrote to do this type of thing was late 90's against Exchange 5.5 using cdo 1.2.
Now it's 2020 and if your running Office365 you should really be using the Microsoft Graph API to do this. So what I've done is create a PowerShell Module (and I made it a one file script for those that are more comfortable with that format) that's a port of the EWS script above that is so popular. This script uses the ADAL library for Modern Authentication (which if you grab the library from the PowerShell gallery will come down with the module). Most EWS properties map one to one with the Graph and the Graph actually provides better information on recurrences then EWS did. Where extended properties where used in the EWS script the equivalent is used in the Graph. (The only real difference is the AppointmentState property which is a strongly typed property in EWS but I had to use the Extended property in the Graph).
Just a couple of things if your new to Microsoft Graph scripts and Modern Authentication that you need to know
1. You need an Approved Azure Application registration to use this (or any script that is going to access the Graph). The Microsoft walk-throughs https://docs.microsoft.com/en-us/graph/auth-register-app-v2 are pretty good at describing how to do this. Specific config I recommend you use
"https://login.microsoftonline.com/common/oauth2/nativeclient" as the redirectURL (this is part of the Suggested Redirect URIs for public clients (mobile, desktop)).
2. Permission for the above
You only need the following permissions for this script to work, Calendar.Read gives you rights to the calendar the account that is being used and Calendar.Read.Shared gives you read access to any calendars that the account being used has been granted access to (eg via delegation, admin portal or add-mailboxpermission).
Then you just need to copy the Application (client) ID guid from the overview screen in the Applicaiton Registration and use that as in the -clientId paraemter in the Export-GCECalendarToCSV cmdlet.
I've included a demo multi tenant app registration as the default in the module that just has these rights which you can use for testing but I would always recommend you create you own.
You can install the module which will give you access to the Export-GCECalendarToCSV and Export-GCECalendar cmdlets from the Powershell gallery https://www.powershellgallery.com/packages/MSGraph-ExportCalendar/ (see the instruction on that page).
Or if you want to take the script and modify it yourself its located on GitHub https://github.com/gscales/Powershell-Scripts/blob/master/MSGraph-ExportCalendar/functions/Export-GCECalendarToCSV.ps1
Simple example of exporting the last 7 days of calendar appointment to csv
Export-GCECalendarToCSV -MailboxName firstname.lastname@example.org -StartTime (Get-Date).AddDays(-7) -EndTime (Get-Date) -FileName c:\temp\Last7.csv