For the last 50 years or so – ever since George Wein and other presenters realized that concert halls and festival stages that went unused during the hot months represented an opportunity – summer has been the key season in jazz. Jazz performers had been to Carnegie Hall and at the Philharmonic, but not until the rise of the jazz festival did the music begin to receive the same kind of cultural acceptance afforded to symphonic music and opera.

These days, summer is the time most musicians leave messages like “see you in September” on their answering machines. That is, when they aren’t playing the New York festivals – and that’s festivals, with an “s.” The event currently known as the JVC Jazz Festival, the heir to the original Newport Jazz Festival, has been the dominant force on the festival scene for 30 years.

There have been attempts to mount a successful alternative fest – mostly led by Michael Dorf of the Knitting Factory – but the only one with legs is the Vision Festival, now in its 10th year. This is an alternative in every way, promoting the avant-garde and the fringe as vigorously as JVC celebrates the mainstream.

For most of the last 10 years, the Vision Fest was at the end of May and JVC was at the end of June. For seven days, you absorbed all the free jazz and experimental music you could; then you rested for a few weeks before getting your annual opportunity to catch the biggest stars of the circuit. This year, however, the Vision Fest and the JVC are being held at exactly the same time. Both start on Tuesday, June 14. The Vision goes until June 19, while JVC has its usual 12-day stay.

I can’t believe Vision and JVC made this decision deliberately, as the effect may be to polarize the jazz world. Still, some of us will want to catch Joseph Jarman performing with two different ensembles at Vision on Thursday (June 16), then at Carnegie the next night for John Pizzarelli and Dave Brubeck (June 17).

In past years, two other festival-style events in the area were geared toward lovers of traditional jazz, Dixieland, and swing: the Jersey Jazzfest and the 92nd Street Y’s Jazz in July. The Jersey fest once was virtually the only musical event (and picnic) worthy of crossing the George Washington Bridge for – one year, as a boy, I actually walked about five miles from the nearest bus stop to get there. That was the only time all year you could get all the Dixieland you could handle: dozens and dozens of bands with banjos, tubas, and clarinets playing “Fidgety Feet.” Likewise, Jazz in July represented artistic director and founder Dick Hyman’s fondness for stride piano and the music of the jazz age – lots of Jelly Roll Morton and Bix Beiderbecke.

Now both events have grown considerably more mainstream: The Sanofi-Aventis Jazzfest 2005 (JVC isn’t the only festival with a corporate sponsor) has left the two beat, straw-hat-and-arm-garter bands behind and is presenting bop piano great Ray Bryant (June 10), along with modernish singers such as Stephanie Nakasian (June 11) and Giacomo Gates (June 12). David Ostwald’s Gully Low Jazz Band (June 12) should satisfy those of us with the Dixieland craving, and Scott Whitfield will lead his fine swing big band (June 12).

The central group, however, is the Statesmen of Jazz (June 11), that rare all-star group that is more than the sum of its members. With a frontline of Kenny Davern (clarinet), Warren Vache (trumpet), and Ken Peplowski (clarinet and saxophone), this is the premier ensemble of the entire pre-modern jazz world.

Jazz in July, now under the direction of Mr. Hyman’s one-time protege, superstar pianist Bill Charlap, is also undergoing modernization. Two evenings this summer will be built around songwriters who go back to the pre-swing era, Hoagy Carmichael (July 26) and George Gershwin (July 19), and there may be some 1920s-style music. But there will also be a focus on bop masters Horace Silver, Kenny Dorham (July 27), and Jim Hall (July 21). And one heavyweight evening of piano, called “Continuum,” will feature five major keyboard stars, starting with Mr. Charlap himself: Hank Jones, Renee Rosnes, Mulgrew Miller, and our old friend Ray Brown (July 20).

One innovation seemingly intended to attract a younger crowd is a “Post-Event Reception for Singles.” That’s a new one for me – I’d better get some shirts to the drycleaners.

The emphasis on festivals doesn’t mean the regular jazz concert and club circuit is out of business. Jazz at Lincoln Center – thank you, Lord – is renting Rose Hall to JVC for a few JVC events (even so, some will still be presented in Carnegie, the Bermuda Triangle of jazz acoustics). In the meantime, its Dizzy’s Club Coca-Cola will continue full blast, with an emphasis on foreign-born jazz prodigies: Russian keyboard virtuoso Eldar Djangirov (June 14-19) and Italian alto saxophonist Francesco Cafiso (June 21-26).

Mr. Charlap, who did two weeks at the Vanguard a few months ago, will do the first week in June (for those who can’t wait until Jazz in July). And the last week in June is manna for trumpet fans, with Dizzy’s saluting Clifford Brown’s 75th anniversary and four star brassmen at the Blue Note, which will present the Dizzy Gillespie Alumni big band.

The Jazz Standard has its own festival like event: the third annual Fred Hersch Duo Series (June 7-12). This is actually more like a sporting event, as the great pianist will pair off with a series of sparring partners: Chris Potter, Ted Nash, Bob Brookmeyer, Stefon Harris, Mark Turner, and the emerging singer Kate McGarry. I wish I could catch ’em all. The rest of the month at the club is appropriately piano-centric: Kenny Werner, Vijay Iyer, Russ Lossing, Makoto Ozone.

Finally, summer is when mainstream clubs seem a little more willing to take chances. For its part in JVC, the Vanguard is presenting six different facets of the multitalented clarinetist, composer and conceptualist Don Byron (June 21- 26). Kurt Elling, who appeared at last year’s Fred Hersch event at Jazz Standard, is headlining an invitational series of his own at Birdland (June 21-25), in which he will sing with three instrumental guests (Bobby Watson, Kurt Rosenwinkle, Stefon Harris) and his all-star male vocal quartet, the Four Brothers, with Jon Hendricks, Mark Murphy, and Andy Bey.

Finally, the Iridium has developed more of a historical bent, what with regular big bands that feature the music of Charles Mingus and the Mingus of rock, Frank Zappa, as well as the Headhunters. Herbie Hancock is no longer with them, but they’re still as funky as ever.