Learn about frost dates. Out in the counties, we can expect a frost anywhere between Oct. 15 and 31, and slightly later in the city. But there are so many shades of frost! There's the just-too-cold-for-the-basil frost (gets down to 40), the nipping frost (whacks the tops of the tomatoes and the edges of the annuals), the heavy-dew-on-the-low-spots-on-the-lawn frost (kills off most of the annuals and any tomatoes you haven't covered at night with a blanket), the white-stuff-on-the-windows frost (makes all the leaves fall off the ginkgo trees in a day), the killing frost (gets down below about 28, and kills everything you're trying to protect), and the hell-freezes-over frost (where the ground freezes solid, and hoses, rain barrels and water pipes burst, figs wipe out, and even the spinach dies). If you care about those things, protect them or harvest them.
Deal with dahlias. Once frost hits and the plants freeze, cut them down to about 6 inches and dig up the tubers. Leave the soil on them, and pack into boxes covered with moist (not wet) sphagnum moss or newspaper, to keep them from drying out and wrinkling. Place in the cellar or unheated garage or shed. If it's really unheated, put them on a high shelf because heat rises.
Enjoy the blues. And the purples. These colors are really coming into their own this time of year with the asters and hydrangeas and sages — even the coleus from which you forgot to pinch the tops. And you should also check out the ephemeral flowers that are suddenly appearing on crab apples and plum and cherry trees as though in anticipation of coming snow.