After completing the task successfully, you have an option to mark it as completed. In the main content area, select the circle available corresponding to the task. The task is marked as completed.
The completed task is automatically added to right sidebar as below.
In the right sidebar, you see this task in the list of completed task on the day when you had created this task, and not on the day when you mark it as complete. (See see details of completed tasks for more details.)
Note: You cannot mark other users' task as completed.
See Number of Completed Tasks
In the left sidebar, you can see your and other member account names. The number of tasks completed by you and other members is available in circle as below.