Make a directory and link it in a node at node creation
Creating directories on the fly in the actual file system is uncharacteristic in Drupal.

The simplest way of attaching photos to nodes would be having an image field with unlimited number of uploadable images. That would show thumbnails of those related images right there when you go to the node page. This is all in core.

If allowing all users to edit event nodes is not an option, you could create an image node type and connect images to respective events with node references or taxonomy, and then create a view listing those images on the node page (views attachment) or on a separate, gallery page.