Source code in the ubasic/4th programming language

Dim @n(12) : Dim @v(12)                ' define both arrays

Proc _DataDays                         ' put data on the stack

For i=11 To 0 Step -1                  ' read them in
  @n(i) = Pop()
Next

Proc _DataGifts                        ' put data on the stack

For i=11 To 0 Step -1                  ' read them in
  @v(i) = Pop()
Next

For i=0 To 11                          ' print all twelve verses
  Print "On the ";Show(@n(i));" day of Christmas"
  Print "My true love gave to me:"

  For j=i To 0 Step -1                 ' show list of gifts
    Print Show(@v(j))
  Next

  Print                                ' next verse
Next
End

_DataDays
  Push "first", "second", "third",    "fourth"
  Push "fifth", "sixth",  "seventh",  "eighth"
  Push "ninth", "tenth",  "eleventh", "twelfth"
Return

_DataGifts
  Push "A partridge in a pear tree."
  Push "Two turtle doves and"
  Push "Three french hens"
  Push "Four calling birds"
  Push "Five golden rings"
  Push "Six geese a-laying"
  Push "Seven swans a-swimming"
  Push "Eight maids a-milking"
  Push "Nine ladies dancing"
  Push "Ten lords a-leaping"
  Push "Eleven pipers piping"
  Push "Twelve drummers drumming"
Return