Just recieved this error:
Could not find any resources appropriate for the specified culture or the neutral culture. Make sure "AjaxControlToolkit.Properties.Resources.NET4.resources" was correctly embedded or linked into assembly "AjaxControlToolkit" at compile time, or that all the satellite assemblies required are loadable and fully signed.
Sometimes .Net errors look a lot more worse than they really are. In this case it had nothing to do with missing DLLs or a bad web.config but rather I simply forgot to register the TookitScriptManager before my CalendarExtender.
<ajax:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server" />