How can I pre-toggle a button in Bootstrap's btn-group?

Question

How can I deploy the Radio Button group and make one of the buttons pre-toggled?

I have a radio-buttons group with days of the weeks values. This one:

Answer 1

I tried the above but wanted a HTML/CSS solution only.

I found a focus based approach would work for Bootstrap 3. Note that autofocus is an HTML5 attribute.

      Mon
      Tue
      Wed
      Thu
      Fri
      Sat
      Sun

I find Bootply is easier for fiddles with Bootstrap.
Here is my test bootply: http://www.bootply.com/cSntVlPZtU