Synchronous After event that fires after a subsite is fully provisioned and the provisioning process has stopped, but does not fire when the root web site of a new site collection is created.
Assembly: Microsoft.SharePoint (in Microsoft.SharePoint.dll)
Available in Sandboxed Solutions: Yes
Available in SharePoint Online
This method can be configured to operate in either synchronous or asynchronous modes. To do so, set the Synchronization attribute on the SPWebEventReceiver object if you are registering by using the object model. If registering by using features, there is a Synchronization element under the Receiver tag.Notes to Implementers
Since an event receiver has to be registered with the site collection before it's handlers can execute, the WebAdding and handlers cannot run when a site collection and its root web site are created.