Q. When I daven maariv after plag and say ya’ale veyavo for Rosh Chodesh Av, can I still eat meat and take a shower before the shekia?

A. Horav Shlomo Miller’s Shlit”a opinion is that when one davens maariv after plag he is considered as having began the next day. Just as with Shabbos, one becomes prohibited in doing melochos after accepting an early Shabbos; so too you would equally be prohibited in all proscriptions that apply to the nine days.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a