Q. Does one need to be machmir for the halachos of the nine days starting from shekiyah or only from nightfall? Maybe one could argue that it is a question of a Rabbinic law where we are often lenient. Thank you.
A. Poskim maintain different opinions. Mogen Avrohom (551: 26) rules that the prohibitions of the Nine Days, including eating meat, begin at the nightfall on the eve of Rosh Chodesh Av. However, he mentions, that some abstain from the beginning of sunset. Pri Megodim (E.A. ibid), Nitei Gavriel (Bein Hametzarim 1: 26: 7), and others prohibit from the shekiah. (see Mishna Berura ibid. 56).
Kaf Hachaim (551: 22) maintains that if one recited the Maariv of Rosh Chodesh early, even if still before the shekiah (after plag hamincha), the prohibitions already apply.
Mekor Chaim (551), Sdei Chemed (Bein Hametzorim 1: 11) quoting Mishkenos Haroim, Taamei Haminhagim (634), maintain that there is a tradition to eat meat on Erev Rosh Chodesh Av and stop before the shekiah, to manifest openly that the prohibition has began. However, Yisod Veshoresh Hoavodo, asserts that one should fast on Erev Rosh Chodesh Av.
Horav Shlomo Miller’s Shlit’a opinion is that one should abstain from the shekiah onward.
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a