Irritating as it may be to find the halls decked with holly before you've properly digested the Thanksgiving turkey, it is a useful reminder that some things are better done in advance, in fact, can only be done in advance.

It is now or never for fruitcakes, plum puddings and sweet-scented pomander balls. It is also a good idea to start gathering ingredients for all sorts of special Christmas dishes.

Following are some places you might find them.

A. Litteri, Inc., 517-519 Morse St. NE (544-0183), is already well-stocked with Italian Christmas cakes -- Panettone and the brioche-style Pandoro, both at $6.95, as well as the flat, nugget cakes, Panforta Sapori, $3.50, and La Florentine Panforte, $5.95. Their cherries in brandy -- to pour over ice cream or press into the top of Christman cookies -- are packed in a glass boot. Italy, of course. Litteri's always has barrels of dried cod at $3.05 a pound to use in the traditional Provencal Christmas Eve dish, brandade: a dried codfish mousse.

The German Deli, 814 11th St. NW. (347-5732), has all the ingredients for stollen tucked away in the housewife's corner: There is a special spice mixture (which also can be used for gingerbread, candied orange and lemon peel, diced or whole, at $1.10 for 4 oz., bitter kernels, 1/2 oz. for 25 cents, bags of whole or ground poppy seeds, or a 12-oz. can of poppy filling for $1.75. The shelves hold tiny, edible wafers to dab your macaroons on so they won't sprawl all over the baking sheet, 98 cents for 100 tiny rounds. There are large rounds, too, for, as the proprietor says, women with little time or no children.

There is a real vanilla sugar for people who've forgotten to tuck a vanilla bean into the sugar bowl. There also are the makings of a Norwegian Christmas: Lutefisk (frozen, presoaked filets) are $10.50 for 2 1/2 pounds; the fresh lingonberries to go with them are $3.95 a pound; Lefse, the pancake-type pastry to wrap the fish or anything else in are $6.80 for three large ones or $4.10 for small.

All of this is displayed in a forest of chocolate Santas, gingerbread houses and gingerbread hearts, barrels of pfeffernuss and marzipan piglets dancing in holiday glee.

The French Market, 1630-1632 Wisconsin Ave. (338-4828), has Buche de Noel, the French cake in the shape of a Yule log, available by order. Last year they cost $9 for a one-foot log, $12 for an 18-inch. This year's prices aren't yet certain, but it's safe to assume they'll be higher. Now also is the time to reserve the Market's fresh foie gras of Boudin Blanc, the white sausage served on Christmas Eve.

Pomanders can be party favors or spicy ornaments on a Christmas tree, so long as you remember to roll them in orris root. Skip that step and they shrivel up in an unattractive fashion.

The Georgtown Coffee, Tea & Spice Shop, 1130 Wisconsin Ave. NW (338-3801) has orris root for $2.25 the jar, as well as cloves enough to cover an orchard of oranges. They also have crystalized orange peel: 5 oz. for $2.99.Though that's enough, they expect to have it in bulk later at a lower price. There is a mulled cider mix, $1 for a packet that will make two gallons, and, a wonderful idea for a centerpiece, or even as a door decoration: 18-inch-long sticks of sweet-smelling cinnamon: 79 cents each.

Now is also a good time to visit the health food stores and buy dried fruits to vary the standard fruitcake/plum pudding mix. Not being partial to citron or candied cherries, I long ago began to substitute honey-dipped pineapple, dried papaya and the like for the more standard fruits. It works.

Lastly, visit the fabric stores and buy up all the red, green and red/green plaid ribbons you think you might need. In two weeks you won't find a yard.