use Date::Manip; 
$input = "June 2006"; 
# these also work:
#$input = "2006 june"; 
#$input = "today"; 
#$input = "June 2006"; 
$thismonth = UnixDate($input, "%B %Y"); 
$lastofmonth = DateCalc($thismonth, "+1 month -1 day");
$dow_lastofmonth = UnixDate($lastofmonth, "%A\n"); 
print $dow_lastofmonth;