Between 17th of Tammuz and the 9th of Av
Because on the 17th of Tammuz the afflictions of the destruction started, therefore some customs (of) mourning (start) from this day, until after the 9th of Av. It is proper for every G-d fearing person to say ''Tikkun Chatzot''1 every day after midnight. One does not get married, even one who has not yet fulfilled the commandment to have children,2 but to get engaged even with the (celebratory) meal, is allowed up to Rosh Hodesh (1st of) Av. From Rosh Hodesh onwards, even though it is allowed still to make engagements, in any event the (celebratory) meal is forbidden, but one can have pastries and the like. A Jew who makes his living from playing music, can play at a non-jew's house in order to earn money, up to Rosh Hodesh. However, from Rosh Hodesh up to after the (9th of Av) fast it is forbidden and on the (fast) day of 17th Tammuz itself, it is also forbidden, and also on the 10th Tevet. Some have the custom of not eating meat or drinking wine from the 17th of Tammuz until after the 9th of Av,3 except on Shabbat4 or at a meal of a commandment.5 (KSA 122:1)
1) Special prayers intended to help correct ones' behaviour and return to G-d.
2) Genesis 1:28: ''Be fruitful and multiply''
3) The whole period is termed the ''Three Weeks'' and the 1st to 9th Av inclusive is known as the ''Nine Days''. Nowadays Ashkenazi Jews usually stop eating meat and drinking wine during these nine days.
4) Customs of mourning are not allowed on Shabbat.
5) Such as the meal after a circumcision or completion of one book of the Talmud and so on.