1) According to the opinion of most authorities, these restrictions (cutting hair, music etc.) begin on the night of the seventeenth of Tammuz.
2) One who goes to sleep on the evening of the seventeenth of Tamuz, and wants to get up before daybreak to eat, must specify before he goes to sleep that he is going to get up to eat before the fast. (As regards drinking, according to the Shulchan Aruch [and the Sephardic custom] it also needs to be specified before going to bed, whereas according to the opinion of the Rem”a [and the Ashkenazi custom] this is unnecessary. It is, however, correct to be stringent and specify even for drinking.)
1) Some have the custom that one who is able to refrain from eating meat or drinking wine in the evening of the seventeenth of Tamuz should do so, nor should he eat meat or drink wine onr the night after the fast, (Kaf Hachaim 564,12), and others write that one need not refrain on the night after the fast. (Mishne Berurah 558,5)
2) On the day of the fast one should be extra careful not to become angry, and if he has any dealings with other people, he should be more calm than usual. (Kaf Hachaim 549,11)
1) One who mistakenly ate in the middle of the fast, and then remembered that it was a fast, or, G-d forbid, ate deliberately, should not continue eating. He must continue fasting until the end of the day. (Mishne Berurah 549,3 568,1)
2) Even one who forgot and ate on a communal fast, may say the prayer of “Anenu” during the afternoon services. (Mishne Berurah ibid)