With Redtail's Calendar, you have the option to set your Completed Activities so that they appear by default.
You can access and modify your Site Preferences from the menu available underneath your Name in the upper right corner:
After clicking Preferences, the Preferences which you can manage are divided into several categories.
The second section you'll see on the Change Site Preferences page is Activity Management:
Click inside the box beneath Default to Show Completed Activities to select Yes.
After making this selection, click Save Preferences 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" in the Actions menu above your Calendar to override this setting temporarily: