To retrieve the month and day of the newest post on your blog via PHP (which you could of course include in a function) is relatively simple – in essence all that you are doing is putting in a call for the WordPress engine to return to you the newest post in the database and then extract the relevant information from there.

The code is as follows:

//Default values for the resulting variables
$newestyear = date('Y');
$newestmonth = date('m');
//Calculate newest Post Date
$args = array('orderby'=>'date','order'=>'DESC','posts_per_page'=>1,'caller_get_posts'=>1);
$newestpost = get_posts($args);

//Extract the date information            
if (!empty($newestpost)){
    $newestyear = mysql2date('Y',$newestpost[0]->post_date);
    $newestmonth = mysql2date('m',$newestpost[0]->post_date);

//Print out the result            
echo "

The newest post of this blog appeared in the $newestmonth month of $newestyear


As simple as that.