Hi Pardo,
Firstly, you'd attach a javascript to TabContainer's OnClientActiveTabChanged event.
<ajaxToolkit:TabContainer ID="TabContainer1" runat="server" ActiveTabIndex="1" OnClientActiveTabChanged="onTabChanged">
Secondly, you shall validate on onTabChanged function.
function onTabChanged(sender, args){
Page_ClientValidate();
}
http://forums.asp.net/p/1239218/2271427.aspx#2271427
Best regards,
Jonathan