Calendar overlays allow you to create a combined view of your Calendars and/or To Do Lists by layering more Calendars or To Do Lists on top. If you assign dates to your Tasks on your lists you can create an overlay to use on your Calendars.
With To Do List Overlays you'll see the tasks show up on your calendar and you can check the tasks off on either your To Do List or on your Calendar. Overlays are only a feature available to paid accounts.
Alternatively, you can use Event icons to add a square next to your events in your calendar. Once you've completed the task on your calendar you can change the square icon to a check mark icon. Follow the steps in the image below:
Once you save your changes you'll see that there's a square icon next to your event in your calendar. After you've finished your task you can then update the icon so that it has a checkmark next to it. See the image below: