title: how to get selected day's key in flutter calendar
body: <p>I have a calendar and whenever I click on a day, I want to get the index(or key I'm not sure) of that day and add an event to it through a text form field? I want to create something like this library  which its name is <code>Table_calendar</code></p>
  <p><img src="https://i.stack.imgur.com/jsnhd.png" alt="" /></p>
  <p>What is the best approach? If you need more information please let me know.
  I have a function that will call when I click on a day. How can I use it
  for putting an event on that?</p>