Crosby, Stills, Nash and Young, who are apparently trying to get back together for a while, are playing a big concert Monday night at the Garden. This looks like one of the summer's few big-deal rock events. You know the scene--waiting in line for hours to get ridiculously expensive tickets, then being part of a huge, half-crazed crowd. Word is that the Monday concert may be already sold out, but that Don Law, the Bill Graham of Boston, is trying to schedule a second one for Tuesday. The problem with CSNY is not that they aren't each good--they're superb rock musicians--but that they're a collection of colossal egos. On the rare occasions when they're getting along with each other, their concerts are terrific, but the rest of the time each one tries to dominate and play long solos, so it all falls apart--all of which is the price an audience has to pay for listening to four superstars playing together. Just hope that they're on, and get in line early.

Falstaff, Verdi's masterful comic opera based on Shakespeare's "The Merry Wives of Windsor" is at the Boston Summer Opera Theater Friday and Saturday nights and next weekend as well. The summer opera is a good institution: for its patrons, it provides low prices and English-language librettos, and for its singers, it's a chance for those who haven't quite made it big yet to show their talents. How good this production will be probably depends on how good the Falstaff character is; it's a challenging role but the keystone of the opera. At St. Alphonsus Theater in Mission Church, near Boston State College, at 8 p.m.

The Duke Ellington Orchestra, under the direction of Mercer Ellington, who must be Duke's brother, is making its first Boston appearance in Franklin Park in Dorchester Sunday night at 8:30 p.m. It's hard to imagine the concert being an entirely happy occasion because Duke's loss will probably be sadly apparent, but the band is still great and the concert is free.

Stravinsky's "Renard" and Poulenc's "Barbar the Elephant," two modern musical classics about animals, are at the Castle Hill Festival Concerts series Saturday night at 8:30 p.m. Besides the music, there's also classical mime by the Pocket Mime Theater.

Miles Davis finishes up his week-long engagement at Paul's Mall on Saturday. You've no doubt heard of Davis: he's the one who invented modern jazz and travels freely through time and space. Beware of a too-short and too-expensive show.

Harvey Brooks and the Fabulous Rhinestones finish Saturday at Sandy's in Beverly on the North Shore. Brooks, the bassist who leads the fast-moving soul band, has the same name as Harvard's dean of Engineering and Applied Physics, and rumor has it they're actually the same person. Check it out.

Larry Groce, an accomplished folkie but nothing particularly special, finishes up his engagement at Passim's in the Square Saturday.

Mongo Santamaria is at the Jazz Workshop until Sunday, and plays fast, rhythmic, big-band Afro-Cuban stuff centering around his own bongo and drum work.