The turn of the season certainly signals change in "Sestina," as it does just about everywhere else. The weather's changing, the temperature's dropping, and especially through the lens of the grandmother, we get the feeling that some sort of bad mojo is coming down the pike. With all the mentions of the equinox and the almanac, we can also assume there is something in the cosmos that has predicted the change that's about to come, and it ain't necessarily gonna be pretty.
The change happened before the kitchen scene takes place, and that's why the grandmother is sad. But hey, things can't get any worse, right?
The change will be good. You can tell by the laughter in the first stanza.