Between the 17th of Tammuz and Tisha B'av
7. From the onset of the month of Av, we minimize our involvement in activities that bring joy ("mema'etim be'simcha") (1). Construction associated with happiness (2) and similarly, construction that is merely a luxury (3), should not be carried out at this time. If one had entered into an agreement with a Gentile to plaster one's house, one should try to offer him a small sum to delay the work until after Tish'ah B'av. If he is unwilling, it is permitted [for him to carry out his work during the month of Av] (4) .
A Jew who has a legal dispute with a Gentile [during the month of Av] should try to postpone the matter, because it is not a propitious time ("riah mazalei"). If possible, he should try to postpone it for the entire month; at the very least, until after Tish'ah B'av.
The moon should not be sanctified until after Tish'ah B'av.
(1) There is a dispute among the authorities as to whether this prohibits ALL activities which bring joy, or whether it just means to minimize those sort of activities (Mishna Berura 551:1 and Sha'ar HaTziyun). We already saw in HY 122:1, that from Rosh Chodesh Av onwards (until Tisha B'av), one may not get married even without a celebratory meal, but one may get engaged without a meal
(2) Such as building a house for one's son who has just become married, or building a vacation home.
(3) That is, it is not essential for one's basic living requirements. Painting, wall-papering and other forms of home decorating should also be avoided. Building a shul or Yeshivah, however, is permissible, as is any construction associated with a mitzvah. Furthermore, all repair work is permissible.
(4) It is permissible because the Gentile is working for his own benefit.