Synchronous Before event that occurs before a new subsite is created, but not 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
The event occurs on the parent Web that is creating the new subweb, since the new subweb does not yet exist. For this reason, the URL-related properties may appear to be null, when in fact these values should be the URL of the parent Web when the parent Web is the root Web. More specifically, the ServerRelativeUrl property in the WebAdding event is the parent Web. On the other hand, this property on the WebProvisioned(SPWebEventProperties) event should be the newly created subweb.Notes to Implementers
Since an event receiver has to be registered with the site collection before it's handlers can execute, the and WebProvisioned handlers cannot run when a site collection and its root web site are created.