jquery表单选择器 如何通过函数来设置预选项

<div data-role="content">

    <form method="post" action="demoform.asp">

      <fieldset data-role="fieldcontain">

        <label for="day">选择日期</label>

        <select name="day" id="day">

          <option value="mon">Monday</option>

          <option value="tue" id="ww">Tuesday</option>

          <option value="wed">Wednesday</option>

          <option value="thu">Thursday</option>

          <option value="fri">Friday</option>

          <option value="sat">Saturday</option>

          <option value="sun">Sunday</option>

        </select>

      </fieldset>

      <a data-role = "button" onclick = "aa()">按钮</a>

    </form>

如何通过函数改变预选项，比如通过js函数使预选项为 Friday