Q. We say that the month of Av has a bad mazal. I have scheduled a necessary medical surgery on the third week of the month, should I postpone it for another month?

A. Shulchan Aruch (O.H. 551: 1) rules that from the beginning of the month of Av one should minimize joy. He adds that it is a month when the mazal is inferior and unfortunate (reiya mazleh), and one should avoid going to trial on this month. Beir Heitev (ibid 2) explains that from the Talmud (Taanis 29a) it would seem that it applies to the complete month. Mishna Berura (ibid. 2) quotes from the Zohar that at least one should abstain until after Tisha Beav.
Kedushas Levi mentions that the month Av is divided into two distinct sections. Part one belongs to Arur (accursed, implied by the alef of Av) and part B stands for baruch (blessed, inferred on the beis of Av), and this lat one contains good mazal.
Chasam Sofer (Droshos 2: p. 331) maintains that a new month begins with Tisha Beav and is named Menachem. Tisha Beav is day one of that new month (See Yerushalmi – Taanis 4: 5, and Yechezkel 26: 1; It came to pass in the eleventh year on the first of the month, that the word of Hashem came to me).
However, Machazik Brocho (Chida 551) and other follow the Beir Heitev’s opinion that the “reiya mazleh” extends for the complete month.
See also question 852; regarding if it is proper for one to salute others from Tu Beav onward with the blessing of Kesiva Vechasima Tova.
Horav Shlomo Miller’s Shlit’a opinion is that in need a person can be lenient after Assara Beav. (Tenth of Av).

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