Q. Can I still shave and hear music on the night prior to the fast (of 17th of Tamuz) since the fasting begins in the morning?

A. Igrois Moshe (O.H.1: 168, 4:112) explains that whether the prohibitions of the three weeks such as shaving and avoiding marriage ceremonies apply already at the night of the seventeen of Tamuz, depends on a disagreement of Rishonim (Riff and Ba’al Hamaor) if we recite “anenu” on the maariv tefilah. He rules that in case of need one may be lenient and perform marriages.

However many Poskim disagree such as Chida in Chaim Shoal (24) in regards to not reciting Shehecheyanu from that night onwards, Maharsham (551: 2) Halichos Shlomo (13: 1) Eishel Avrohom (551) et. al.

Horav Shlomo Miller’s Shlit”a opinion is that indeed one should begin the three week mourning period and all the traditions involved from the night of the Shiva asar betamuz onwards. (In case of great need, Sheva Brochos or Pidyon Haben, one should ask a competent Rabbi)