'Asarah BeTebeth (the fast of the 10th of Tebeth) and Shib'ah 'Asar BeTammuz (the 17th of Tammuz) are two of the public fasts where everybody is obligated to fast, but are among the fasts that start in the morning and not the night before. They start in the morning before daybreak and end, of course, after Seth Hakkokhabim (after the stars come out).
Because of that, there are those who feel that the night before is not connected with the fast day in any way. That is a mistake. In particular, if they arrange celebrations on the night before the fast, with dancing and music, this is a very grave error of judgment.
People should be warned about this and told that it is not appropriate to have music and dancing the night before a fast. Also, if you are invited to such an event, you must know that it is the wrong thing to do.
(See Ben Ish Hai, Shanah Aleph, Debarim, Oth Heh)
