Saturday nights in Washington are feast time for people who listen to jazz on radio.

Many people are torn about where to turn on the dial.

What do you choose - Yale Lewis' comforting modern sounds on WETA-FM, Byron Morris' more contemporary offerings on WPFW-FM or Russell Williams' avant-garde servings on WAMU-FM?

And for the everyday jazz listener, there are healthy doses of jazz throughout the week. WGTB-FM, the Georgetown University station, has six different jazz shows. WAMU-FM, owned by American University, devotes 14 1/2 hours to jazz each week. Most of the jazz on WETA-FM comes in one chunk - the seven hours Lewis does on Saturday nights.

Leading the pack is WPFW, the newest radio station in town, which programs a healthy 104 hours of jazz a week.

Washington is in the midst of a resurgence of jazz on radio. It wasn't stations - WMAL and WHUR-FM. On radio was confined to two local always this way. Five years ago jazz Since then, however, the latter, the Howard University station, has opted for more programming of the black disco sound.

The local renascence is part of a larger national jazz resurgence."There has been a steady wave that's been getting bigger and since 1967 or 1968," says Ira Sabin, who monitors tht jazz trends for his publication, "Radio Free Jazz."

He continues: "The cross-over or hybrid jazz have been getting more airplay. So when the cross-over type music infiltrates the airwaves, it paves the way for more straight-ahead jazz, the acoustic type."

The jazz resurgence has gone beyond radio to spark increased attendance at night clubs and concerts and more record sales. Jazz on radio, in Washington and elsewhere, is found primarily on the FM band - and that means listener memberships, not commercials, count for station support.

FM jazz is much more experimental than the music on AM radio. Jazz purists, particularly the young, wouldn't pause to tune in Felix Grant, who might be playing Freddie Hubbard's disco records, when they can listen to an FM announcer playing a 45-minute clanging, dissonant solo by piaist Cecil Taylor or six different versions of "Star Dust" by various performers.

A. B. Spellman, author of "Black Music. Four Lives" and an official with the National Endowment's Expansion Arts Program, delivers radio essays in his show for WPFW.

In a recent one-hour feature on Taylor, Spellman explained the roots of the musician's career and music, and he made useful comparisons of Taylor with older pianists such as Art Tatum and Thelonious Monk.

He has also aired programs analyzing the music of trumpeter Dizzy Gillespie and avant-garde saxophonist Anthony Brazton.

Spellman's delivery is choppier than an AM announcer's, but the content of his show is like seminar material - for the uninitiatied and musically sophisticated.

Less esoteric but no less dedicated are announcers like Yale Lewis, Rusty Hassan and Ken Steiner.

Lewis, a 33-year veteran in the business, has built up a considerable following for his Saturday night prgram on WETA-FM, "Jazz Plus." But he thinks his show would be more effective in a weekday time slot.

"Why not put me in a different time period?" he asks in his honeyed deep basso. "I can't do anything on Saturday. Lots of people arent staying at home to listen to radio. They're out there in the streets partying!"

Lewis is likely to play plenty of soothing Saturday night music - Joe Williams or Arthur Prysock vocals, ballads by tenor saxophonist Gene Ammons and oodles of big band sounds.

For his Sunday afternoon listeners to "The New Thing Root Music Show," Hassan will usually feature loads of standard modern jazz - Charlie Parker and Dizzly Gillespie, Sonny Rollins and Bud Powell, Clifford Brown and Lee Morgan.

One of his favorite programming approaches is to feature several versions of the same piece - maybe "Star Dust" or "Tenderly."

At 24, one of the younger jazz announcers, Steiner has ondness for contemporary jazz - the Art Ensemble of Chicago, Anthony Brazton, Muhal Richard Abrams, Sam Rivers.

And he's apt to feature them in two different shows he does for WGTB and WPFW. In recent months, Steiner has shown a capacity for broadening the content of his shows to include older jazz, even reaching back to the '20s.

Most jazz announcers see their roles as educational and as a labor of love. The announcers on the university stations and WPFW are not paid. Lewis receives a modicum salary.

"Its always been difficult selling the idea of jazz," says Bill Cerri, a former jazz announcer who now produces classical music shows for WETA-FM. "There was never any commercial acceptance of this music."

Why?

Cerri says the jazz audience is not as vocal as pop and classical music listeners. "Whenever we have fund-raising marathons at WETA, the classical audience makes its contributions," notes the announcer. "But the jazz audience falls short. Many can't afford it - and those who can don't contribute."

Until jazz listeners become more vocal, says Cerri, radio stations will not feel the need to program much jazz.

And that means we're in store for more declines and resurgences.