Within index.php in the postmetadata part looking something like this: <p class="postmetadata"> <?php _e('Filed under:'); ?> <?php the_category(', ') ?> <?php _e('by'); ?> <?php the_author(); ?><br /> <?php comments_popup_link('No Comments »', '1 Comment »', '% Comments »'); ?> <?php edit_post_link('Edit', ' | ', ''); ?> Published <?php the_date('F j, Y'); ?> </p> You need to look for something like that in bold. You may need to remove this from other files that are using most of the code from index.php too.
If you are using Word Press, you can do the same thing under settings and Permalink. Just beware taht if you change that, either the way you did or through settings, any old posts will change the URL: and any that are already indexed will have to be reindexed.
BTW you need to remove same thing from single.php also otherwise post date info appear there for single page view. DON.