On the last of October (6)
Sandburg knows that we typically mark the seasons with special holidays and celebrate them on specific days. So the seasons and our traditions go hand in hand.
Singing ghost songs And love to the harvest moon; (10-11)
It's all one big, happy family here: the traditions and nature ("harvest moon"). So it seems the best way to celebrate holidays in Sandburg's poem is to get outside and give that DVR a rest.
I am a jack-o'-lantern With terrible teeth (12-13)
Nothing says tradition better than a glowing "jack-o'-lantern" to ward off those evil Halloween spirits. But Sandburg cares less about the evil part and more about the fun the kids have with the pumpkins, since they're only fooling after all.