With Redtail's Calendar, you have the option to set your Completed Activities so that they appear by default.
To set this up, click on the Manage Your Account menu and select Change Site Preferences:
The second section you'll see on the Change Site Preferences page is Activity Management:
Click on the menu box to the right of Default to Show Completed Activities to select Yes:
After making this selection, click Save in the upper right hand corner of the Activity Management section and you will have successfully changed this setting. Note: when viewing your Calendar, if you have this as your default setting, you can click "Hide Completed Activities" in the Actions box to the right of your Calendar to override this setting temporarily: