[jQuery] JQuery bigginer

Hi I am learning JQuery and Javascript so asking this simple question.
I have a text field which has default value as Day. I want when user
focus on this text field then it become empty. How can I do this using
JQuery?
<s:textfield name="day" cssClass="textbox1" size="3" id="dayOfBirth"
value="Day" required="true" maxlength="3"/>