It's all about the setting in "Theme in Yellow." The imagery of autumn and all of its changing colors are the qualities that really bring the pumpkin-speaker to life. Without the "orange and gold clusters" and "harvest moon," the poem would lose much of its spirit. And since it's Halloween, we really need a setting that helps put us right in the experience ourselves. After all, it wouldn't really make sense to have a talking pumpkin in the middle of winter, right?
It's all about the pumpkin's world here and what it looks like. So the speaker is careful to paint the setting of this autumnal world in a highly vivid and imaginative sort of way that puts us right in the front seat. The time, as far as years are concerned, doesn't matter so much, because autumn tends to look the same no matter if we're talking about the nineteenth century or the twentieth. Of course, the Halloween tradition has been around for quite some time, so the focus is on the scenery of the setting, rather than the time.