Add CHANGELOG entry and update the guide

activerecord/CHANGELOG.md

 ## Rails 4.0.0 (unreleased) ##
 
+*   Allow before and after validations to take an array of lifecycle events
+
+    *John Foley*
+
 *   Support for specifying transaction isolation level
 
     If your database supports setting the isolation level for a transaction, you can set

guides/source/active_record_validations_callbacks.md

@@ -995,6 +995,25 @@ class User < ActiveRecord::Base
 end
 ```
 
+Callbacks can also be registered to only fire on certain lifecycle events:
+<ruby>
+class User < ActiveRecord::Base
+  before_validation :normalize_name, :on => :create
+  
+  # :on takes an array as well
+  after_validation :set_location, :on => [ :create, :update ]
+
+  protected
+  def normalize_name
+    self.name = self.name.downcase.titleize
+  end
+
+  def set_location
+    self.location = LocationService.query(self)
+  end
+end
+</ruby>
+
 It is considered good practice to declare callback methods as protected or private. If left public, they can be called from outside of the model and violate the principle of object encapsulation.
 
 Available Callbacks