jquery datepicker predefined date

Question

This is my textbox where the datepicker is attached to:

<asp:TextBox runat="server" ID="StartMonth" AutoPostBack="true" class="month-picker" />

and here is how I attach the datepicker. (I have multiple TextBoxes where the datepicker should be attached to, so I used the class attribute instead of the id.)

$(".month-picker").datepicker({ dateFormat: 'MM yy', changeYear: true, yearRange: '-9:+9' });

When I click into the textbox where the text is "August 2012" it should set the pre-selected date of the datepicker to this date. How do I do that?