When you add share point calendar control to your custom application pages, there will be lot of style issues. In order to resolve this,
a.Navigate to the page iframe.aspx page located in C:\Program Files\Common Files\Microsoft Shared\web server extensions\14\TEMPLATE\LAYOUTS.
b. Add the following css file reference manually
<link rel="stylesheet" type="text/css"href="/_layouts/1033/styles/datepicker.css"/>