(asp.net) Calendar extender problem

I have a calendar exter from ajaxcontroltoolkit in the second panel. On the first panel I have a button"next", so when I press it the first panel visibility is set to false and the second's panel visibility is set to true, than I click on my textrbox with data and surprisingly the calendar appears, but not as expected, it has no nice appearence as it should be by default. What is the problem?

protected void btnNext0_Click(object sender, EventArgs e){Panel0.Visible = false;Panel0.Visible = true;} The most interesting that on the first panel the calendar works just fine.ups, I forgot to say that the panels are located in UpdatePanel, if it plays here any importance.