Trigger an action at a specific time using Trigger and Rules I would like to run an action once a specific $_SESSION variable is set. In both Trigger and Rules, I do not find a description on how to do something like that. Can anybody help me, or at least hint at a website explaining this? 

I am looking for an easy way to initiate a call at a specific time (think of as a Drupal alarm clock); my idea was to put the time in a $_SESSION variable and fire the alarm clock once $_SESSION['alarm'] == strtotime("now").

In case that this should just not be possible: I would also be happy with a trigger that fires whenever I go on a certain page.
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"