You need to edit the index.php in the template folder of your theme.
this is the code:
<small>Published by <?php the_author() ?> on Tagged <?php the_category(', ') ?></small>
if you want to display the date just replace it with this:
<small>Posted <?php the_time('F jS, Y') ?></small>
Of course you can change the word "Posted" with what ever you like or even get rid of it and display just the date.
However notice that there is already a date showing at the bottom right corner of the post. this is accomplished by this line of code:
<span class="date"><?php the_time('F jS, Y') ?></span>
you can remove it so to avoid the double date.