You may have seen sometimes one or many of the calendars which you manage will show the Mail Tip as This item will expire in somedays. To keep this item longer apply a different retention policy as shown in the below snip.
What this is telling you is, A Retention Policy is applied on the calendar you are managing, which removes/archives the calendar items periodically based on the policy settings. This is done to automate the mailbox cleaning by removing the mails/attachments and clear the mailbox space for the future calendar bookings.
You can configure the policy to specify how long a message can remain in the mailbox folder and the action to take when the message reaches the specified retention age. When a message reaches its retention age, it’s moved to the user’s In-Place Archive or deleted.
You can set the retention age as per your Organizational requirement/policies. To check the available Retention policies, run the below Cmdlet in PowerShell
To see the retention policy applied on the calendar, Run the below Cmdlet
Get-Mailbox “Mailbox Name” | FL RetentionPolicy
If you want to know more on the Retention policy, retention age, and retention action, run the below Cmdlet
Get-RetentionPolicy "Default MRM Policy" | FL