1) One may not arrange dances and festivals in the three weeks. (Nowadays it also forbidden to listen to recorded music). (Mishne Berurah 521)
2) A Jew whose income comes from playing music, may play for non-Jews until the first day of Av, except on the seventeenth of Tamuz itself (and the tenth of Teves), when one may jot. (Kitzur 122, Ben Ish Chai Devarim 12)