Full version:
<? echo date('F Y'); ?>
Short version:
<? echo date('M Y'); ?>
Here is a good reference for the different date options.
update
To show the previous month we would have to introduce the mktime() function and make use of the optional timestamp
parameter for the date() function. Like this:
echo date('F Y', mktime(0, 0, 0, date('m')-1, 1, date('Y')));
This will also work (it's typically used to get the last day of the previous month):
echo date('F Y', mktime(0, 0, 0, date('m'), 0, date('Y')));
Hope that helps.