Task Widget is an extension for GNOME that displays tasks next to the calendar widget. It integrates seamlessly with GNOME Online Accounts and a number of GNOME applications, such as Evolution and To Do. By design, it's not meant to be a replacement to those applications as its functionality is limited to viewing tasks and setting their completion status.
To get started, you need to download and install the extension. There are a couple of ways to do that, visit the Installation page and read more about it.
If you found a problem or have a feature suggestion, please report the issue to the bug tracking system.