With the Twitter module, it is easy to post a message to Twitter when a node is created or updated. However, to post the message at any other time requires a bit more work.
Twitter contains a sub module - Twitter Actions. This sub module enables users to post a message to Twitter with any custom business logic. This document will show you how to take advantage of Twitter Actions to post a message to twitter when a node is published. This is a common occurrence in Drupal. Users may want to create a lot of unpublished nodes, and then wait for an admin to approve them, before they are published.
On the Modules page, enable the twitter_actions module.
Step by step guide:
Option A - Use the Rules module:
Set up the following logic.
ON event - Content is going to be saved
IF NOT condition - Unchanged content is published
AND condition - Saved content is published
DO action - Post a message to Twitter
Option B - Build a custom module
1. Go to Site Configuration->Actions, and create a new advanced action called "Post a message to Twitter..."