Our custom is to avoid listening to music during the three weeks from Shiba Asar Be’Tammuz through Tisha B’Ab, and to avoid eating meat from after Rosh Hodesh Ab through Tisha B’Ab. During the week of Tisha B’Ab, from Mosa’eh Shabbat until after the fast, we avoid shaving and haircutting.
The prohibition against eating meat begins after the period of Ben Ha’shmashot – or approximately 15-20 minutes after sundown – on Rosh Hodesh Ab. The prohibition does not begin at sundown, but rather after the period of Ben Ha’shmashot. The period of Ben Ha’shamshot is a time which cannot be definitively classified as either day or night, and therefore, as we deal here with a custom, as opposed to a strict Halachic prohibition, we may allow eating meat during this period. The Mishna in Masechet Ta’anit forbids eating meat during the final meal before Tisha B’Ab, but allows meat until then. It is only by force of common custom that we apply this prohibition already from the second day of Ab. Therefore, one who wishes to eat meat after sundown on Rosh Hodesh Ab may do so, as long as he has not yet recited Arbit, and he finishes by the end of Ben Ha’shmashot. For example, if sunset on Rosh Hodesh Ab occurs at 8:20, one may eat meat until around 8:35.
By the same token, those who abstain from wine during the Nine Days may drink wine throughout the period of Ben Ha’shamshot, even after sundown.
Summary: The prohibition against eating meat during the Nine Days takes effect approximately 15 minutes after sundown on Rosh Hodesh Ab.