When summer thaws out the city, serious Chicagoans metamorphose into carefree campers, and workaholics shed their pin stripes for play clothes. Cocktail-hour conversations turn from mortgage rates to golf handicaps, and nobody works late.

On the sunniest afternoons, thousands of grown-ups sneak away from work to cheer on their beloved Cubs at Wrigley Field, the only major league baseball park where games can be called on account of dark. The well-tended parks and clean beaches fronting Chicago's 17-mile-long Lake Michigan shoreline offer sunbathing, public tennis courts, volleyball and hot dog stands.

But there's more to summer here than admiring the skyline from the surf. In Chicago, summertime is party time, and City Hall dips into a $4 million pool of hotel tax revenues to underwrite a season's worth of free music, ethnic festivals and street fairs.

From now until Labor Day, there will be parties every weekend, from one end of the city to the other. Most events are free, although a few collect a $5 fee at the gate.

From free symphonic music to a week-long jazz festival, from Italian sausages to Cajun food, August in Chicago offers something for every palate. Among the possibilities: Irish Fest, Aug. 1-3, noon-11 p.m., Olive Park, admission $5. Scheduled performers include dancers doing Irish jigs, fiddle players and "leprechauns" telling old-fashioned folk tales. Grant Park Symphony Chorus, Aug. 2, 8 p.m.; Aug. 3, 7 p.m.; Petrillo Music Shell, Grant Park. Take a blanket and a picnic and relax on the lawn while the music plays. The scheduled program: Spohr, Symphony No. 9, "The Seasons"; Orff, "Carmina Burana." North Halsted Market Day, Aug. 2-3, noon-8 p.m., 3200-3600 North Halsted St. One of the recently gentrified sections of town, North Halsted boasts nouvelle cuisine, antiques, vintage clothes and a couple of million carats of rhinestone jewelry. The annual market days bring merchants and their wares out of the shops and onto the sidewalks. Chinatown Summer Fair, Aug. 2-3, noon-8 p.m., Cermak and Wentworth avenues.More than 20 Chinatown restaurants will offer food along the streets. Entertainment includes martial arts masters demonstrating their skills, traditional Chinese dancers, magic acts, American rock 'n' roll and Chinese fireworks. Lyric Opera Center for American Artists, Aug. 6, 8 p.m.; Aug. 8, 8 p.m.; Petrillo Music Shell, Grant Park. Strauss' "Vienna Life". 29th Annual Gold Coast Art Fair, Aug. 8-10, noon-10 p.m., Rush Street between Chicago Avenue and Cedar Street. With more than 600 painters and sculptors exhibiting their work, this fair promises to be more than a summer arts and crafts show. In the heart of the downtown nightlife district, the fair is a short walk from downtown hotels. Pan American Festival, Aug. 8-10, noon-11 p.m., Olive Park, admission $5. Three nights of hot music, spicy food and fireworks. Grant Park Pops High Class Brass, Aug. 9, 3:30 p.m.; Aug. 10, 7 p.m.; Petrillo Music Shell. A night of trumpet music on the lawn, with experiments in pop, jazz, rock and Latin musical traditions. Sculpture Chicago '86, Aug. 12-Sept. 19, 24 hours daily, Harrison and Wells streets. Sculptors who build big things use cranes, cement mixers and welding torches as comfortably as painters wield brushes. Sculpture Chicago is in its fourth year of offering art fanciers the chance to see sculptors work al fresco with heavy machinery. Six artists will be in residence this year, and the seven-acre site displays works from past shows as well. Grant Park Symphony, Aug. 13, 8 p.m.; Aug. 15, 8 p.m.; Petrillo Music Shell. A night of picnicking on the lawn, to the strains of cello music, featuring Debussy, "Nocturnes"; Honegger, Cello Concerto; and Roussel, Symphony No. 3. 15th Annual Broadway Art Fair, Aug. 15-17, noon to dusk, 2800-3400 N. Broadway. One of the up-and-coming Chicago neighborhoods, the New Town area is home to trendy boutiques and hot dog stands, poster shops and Chinese takeout kitchens. There are always good reasons to visit this area; the art fair is icing on the cake. Festa Italiana, Aug. 15-17, noon-10 p.m., Olive Park, admission $5. Fireworks, bingo and more than 50 vendors selling Italian food and gifts, plus music ranging from Frankie Valli and the Four Seasons to Italian balladeers. Munch on zeppole, pizza, stuffed rice balls and gelato and shop for brightly colored ceramic pasta plates and soup tureens. Grant Park Symphony Chorus, Aug. 16, 8 p.m.; Aug. 17, 7 p.m.; Petrillo Music Shell. Berlioz' "Requiem" on the lawn. Buckingham Fountain Art Fair, Aug. 17, 8 a.m.-6 p.m., Grant Park. A short walk from the Petrillo Music Shell, Buckingham Fountain has been a Chicago landmark since its dedication in 1927, with bronze seahorses at its base and three pink marble basins. Colored light displays flash nightly from 9 to 10 p.m. The central jet spurts to 135 feet. Grant Park Symphony, Aug. 20 and 22, 8 p.m., Petrillo Music Shell. Dvorak, Violin Concerto; Tchaikovsky, Symphony No. 5. Aug. 23, 8 p.m.; Aug. 24, 7 p.m. Beethoven, "Egmont" Overture, Piano Concerto No. 4, Symphony No. 5. Chicago Jazz Festival, Aug. 27-31, 6:30-10:30 p.m., Petrillo Music Shell, Grant Park. The last and biggest party of the summer, the eighth annual Jazz Festival boasts 28 acts this year. Headliners include Sarah Vaughan, Cab Calloway, George Benson, Art Blakey and the Jazz Messengers, and artists imported from London, Paris and Puerto Rico.

Like the annual June blues festival, the jazz festival calls itself the largest free concert of its kind in the world. Crowds of 25,000 a night came and listened last year, and there was room in the park for everyone and their blankets. New Orleans-style food booths will sell Creole and Cajun snacks.

To help visitors plan their time and choose among the festivals, the city has a 24-hour recorded announcement: (312) 744-3370. The Petrillo Music Shell, in Grant Park, is a short walk from most downtown hotels. All the other festivals can be reached by bus or taxi. For information, call the Chicago Transit Authority: (312) 836-7000. Laurie Kalmanson works in The Post's Chicago bureau.