For folks who delight in chamber music, this weekend offers some nearly irresistible doings: Mozart sonatas for piano and violin at the Library of Congress, and the much- acclaimed Guarneri String Quartet in an eclectic program at the Kennedy Center's Terrace Theater.

* Pianist Malcolm Bilson and violinist Sergiu Luca come to the Library of Congress' Coolidge Auditorium -- Friday at 8 and Sunday at 3 -- to play eight sonatas written from 1778, when Mozart was 22, to 1788, when he was a ripe 32.

Luca and Bilson have been using period instruments -- a 1750 Gagliano violin and a replica late-18th-century fortepiano, tuned about a half-tone below the modern mode -- to perform 20 Mozart sonatas for keyboard and fiddle. For a chance to get a seat at these last free concerts in the cycle, you're advised to come about an hour early on Friday, and half an hour early on Sunday. Call 287-5502.

* The Guarneri String Quartet -- Arnold Steinhardt and John Dalley on violin, Michael Tree on viola and David Soyer on cello, who are marking their 18th season together -- take time out from being artists in residence at the University of Maryland to present works by Mendelssohn, Stravinsky, Puccini, Wolf and Beethoven at 7:30 this Friday in the Terrace Theater. Tickets $12. Call 254-9895.