Attach different CSS styles on a node basing on publishing date or other PHP properties I want to attach a different CSS style to a node, basing on the node publishing date or some PHP properties.
What would be the best way to do this?
You are asking the wrong question. The question you should have asked is "how do I add a class on a node depending on some properties?". Using different set of CSS files to control styling a an element is just a no-no.

The answer to the correct question depends on whether you are targeting Drupal 6 or Drupal 7:


On Drupal 6, you will generally have to implement the logic your own node template;
On Drupal 7 (or on Drupal 6 if you are using an intelligent base theme like Zen), you would implement a hook_preprocess_node() in your theme and add classes to the 'classes_array' variable