#!ruby -pardate
$_=(t=Date.parse$F*'/').strftime("%B %Y").center(20)+"
Su Mo Tu We Th Fr Sa
"+' '*t.wday*3
$_+='%2s'%t.day<<32-(t+=1).cwday/7*22until/(
.*){8}/
sub(/ +1.*
.*\Z/,$/)
Note that non-ascii characters in the above source code will be escaped (such as \x9f).