Calendar problems in IE 8 are fixed in latest Rad Controls release. The latest Rad Controls release is included in tSitefinity 3.6 SP1. So if upgrade to Sitefinity 3.6 SP1 most probably you will not have the problem in calendar in IE 8.
If you don't want to upgrade now, you can read Compatibility and IE8
article and make IE 8 behave like IE7.
the Telerik team