Displaying the day on the calendar from the textbox generated

Displaying the day on the calendar from the textbox generated

may i ask on how to display the day (Monday, tuesday,etc) from the textbox
which had a datepicker javascript, into the textbox which would echo the
day from the datepicker.
example: textbox1(datepicker, when the user selects september 12 2013, it
would echo Thursday to textbox2) textbox2 = Thursday. Thank you.