Calendar keeps track of the state of To Do items: whether they're completed or pending. In the To Do List dialog box, you can choose to display all To Do items, only completed items, or only pending items.
If you make a To Do list and then want to check off an item on the list, use one of the two methods described below.
Choose To Do List from the View menu.
Click the check box beside the To Do item you've completed.
Click Apply or OK.
This changes the item's state to completed.
To return the item's state to pending, click the check box again and apply your changes.