Trigger an action at a specific time using Trigger and Rules
Why are you interested in the $_SESSION variable? What is the bigger action you are trying to accomplish?

The easiest way to fire any event when a user visits a page in Drupal is too use hook_menu and create a custom callback. 

If you are trying to fire an action at a specific time you can setup a cron job to go off at the time you desire.  Drupal 7 core has a built in cron mechanism you can tell to go off at certain time increments /admin/config/system/cron . If you don't have access to your server to setup a cron job and want it to go off at a specific time once a day (ala an alarm clock)  you can use one of these modules Advanced Cron Modules

In order to respond to a cron job you can use hook_cron or using a cron trigger at "/admin/structure/trigger/system"