With the supermarket bins piled high with such exotica as kiwi fruit and passion fruit, papayas and mangoes, with strawberries and melons available almost all year 'round, with everything we ever wanted just a short drive away, it is nice now and then to pay tribute to truly seasonal items, the things that demand you eat, drink and be merry now, because tomorrow they ain't gonna be here.

Rhubarb is one, and, though the pink stalks look rather tired by the time they get to market, they make a pie worth waiting a year for.

Bock beer is another, brewed and sold only for a short spring season and the occasion of many an annual bock-beer festival. Bock is a dark and heavy beer whose flavor comes from an extra-long roasting of the malt.

It is not, myth to the contrary, the leavings at the bottom of the vat, served up each spring when the vats are cleaned. Bock and the festival in its honor originated in Einbeck in Germany, hence einbeckerbier, and, finally, bock. The baste for the dark brew and the desire to celebrate its appearance undoubtedly came to this country with the German immigrants.

For years in Chicago, a city with a large German population, a harbinger of spring that preceeded the first crocus was the Germania Club's annual bock-beer festival, with kegs of beer flowing, tubas and accordions warring away and always a live goat in attendance Bock also being the German word for ram).

In Washington, Old Europe, 2432 Wisconsin Ave. NW, recognizes the bock-beer season and, though their festival was held earlier, they should have bock on tap through the end of the month.

For a full-scale celebration, you'll have to do it yourself. Schaeffer, Stroh's and Pabst make a bock beer each spring which should be available any place that stocks those labels. Polka music on the phonograph can serve in lieu of an accordion or tuba player -- just to provide an oom-pah-pah sound to wave your steins to.

And, of course, there must be a table piled high with German sausages, rye breads and mustards. The German Deli, 814 11th St. NW, is a wonderful place to stock up on the accompaniments to bock beer. They have a delicious Westphalian ham for $4.30 a half-pound (all prices quoted are for a half-pound), salamis like the German cervelat for $3.30, hard salamis at $2.90, rugenwalder for $3.30, Baurn at $2.90 and pepper-specked thuringer at $2.45.

They have headcheeses, plain, sour or smoked, endless jars of mustards and baskets full of rye. They also have Bundnerfleisch, the air-dried beef, at $9.25 a half-pound, which sounds like a lot except you slice it very, very thin so a half-pound goes a long way.

The German Deli also can provide your buffet with sauerkraut or red cabbage, each $1.20 a pound. If you want to get enough to serve lots of people, you'll have to bring your own containers. They can only cope with up to 5 pounds. And if you haven't got a polka record, they have those, too.