Google Tasks provides a powerful link between Google Mail and Calendar and can be easily accessed from both Gmail and Google Calendar.
When you click on a date in your Calendar, the Create Event window opens where you can add basic details about your new event. You can also switch that window to Create Task where you can enter the information regarding your new task. At the top of the window, just click the Task link and the window will change.
Enter the task, jot down some notes, and then click Create Task.
You will notice that the task will be added not only to the date you assigned it to, but also to your Task list in the right sidebar.
When added to a calendar date, you will notice a checkbox next to the task name. When you complete the task, you don’t need to check it off in the sidebar. Rather, you can check it off within Calendar itself. When you do, the task will be crossed off both in the calendar and the sidebar.
Clicking on the task in Calendar will open the window with the Task details. You can add notes, check off the task, change the due date, move it to another list, or delete it altogether.