i believe the right hook for displaying the actual column content (each line) is
manage_{$post_type}_posts_custom_column
so change in your code:
add_action( 'manage_event_custom_column', 'event_date_column_display', 10, 2 );
to:
add_action( 'manage_event_posts_custom_column', 'event_date_column_display', 10, 2 );