In general the restrictions placed on these days are based on those placed on mourners, but different authorities employ vastly differing levels of stringency for the time between the fasts. The general rule is that the lowest level of stringency is given to "the three weeks" (after the 17th of Tamuz until the beginning of Av). The second level being from the beginning of the month until the fast itself, and the third level being the day of the fast.
The first level restricting people from lighter forms of enjoyment, such as shaving and swimming, listening to music and weddings. The second restricts us from eating meat and drinking wine, showering and washing clothes. And of course the actions to be performed on the day before the fast itself are to be kept in a much more mournful demeanor than any time previous to it.
These are the customs as laid out by Ashkenazi educational institutions. As with many of their customs, I knew early on that many of them were of dubious origin in regards to traditional Judaic literature. I also knew that Sefaradim traditionaly did shave during "the three weeks", and were sometimes looked down upon by their European brethren for breaking halachic boundaries during these days.
I should mention that none of this would be of such importance to me (not that it's not important...I'm just not sure I would generally be speaking about hilchot Bein Hamitzarim on the blog) if not for the fact that I was considering attending another of my friend's concerts he's putting on next week (in Harlem no less. Perhaps that's why it's free. Which is of course my incentive to be going), and was considering the halachic permisibility of such an engagement.
In regards to his (the singer's) halachic status, I'm reminded of what Rabbi Avraham Gombiner (מגן אברהם) comments on the Shulchan Aruch (very loose translation) "It is permissible for a Jew who earns his livelihood from music to play for non-Jews during the three weeks. Even if he plays enjoyable music, he himself is not enjoying it that much because "it's his job" to play"(תקנא:ב).
Though it's quite obvious that rabbi Gombiner's rulings are based on a much earlier Ashkenazi tradition that veered from the Sefaradi one a long time ago. What are the earliest sources for these "three weeks"?
Well, as we already established, the fact that there is an interim period of mourning between the "17th" and the "9th" is obvious, both from the haftara placings and other early Talmudic and post-Talmudic passages. But what of actual restrictions and commemorations during these times? On this point there is recorded, even in the Shulchan Aruch itself, customs of pious individuals to refrain from eating meat, and of some to even fast daily, from the 17th of Tamuz. Yet the same custom is recorded about others from the beginning of Av and from the week in which the 9th falls.
Upon a quick realization that it is beyond my current level of patience to record here the slow evolution of halacha between Early German Jews and Early Spanish Jews, I will suffice by saying that for anyone who does even a superficial reading of the sources, it is clear that there is no "traditional" or Sefaradi observance of the "three weeks". In fact, there is no concept of the "three weeks" in the Sefaradi halacha system. In fact, neither is there a concept of the "nine days". Both are European inventions. In our understanding the time after the 17th of Tammuz is in fact a mournful time, but that isn't really manifested in action. The first hint the Talmud gives us that mournfulness should become manifest in our actions is the statement that "with the coming of Av we should reduce in joy" (משנכנס אב ממעטין בשמחה), but that too is mainly manifest on the week in which the 9th falls. In the communities of The Pious Men of Germany (חסידי אשכנז), many more costoms and restrictions were added to be observed during these times, and those customs are what later Ashkenazi halachic literature is based on, but again, that has no bearing on Sefaradi halacha.
It should also be mentioned that no early source records any restriction on playing musical instruments, either in early Ashkenazi or Sefaradi traditions. Does this mean I can go to the gig? Well, basically "yes", since all we're speaking of here is just the arena of customs, not halacha per-se, but it is still worthwhile to see what some Ashkenazi authorities make of the music prohibition today.
In his essay, Rabbi Eliezer Melamed (of the Beit El yeshiva) records some of the prevalent moods of halacha regarding music during the "three weeks" among Poskim today. He comes to the conclusion that there are, in fact, three categories of music in this respect: Joyous music, neutral music, and sad music. He allows neutral music during the "three weeks" and sad music during the "nine days". In that case, while not particularly in the spirit of the times, though also not quite prohibited, it would be allowed, even according to the much stricter Ashkenazi halacha, to listen to neutral music, which I feel is the type of music I plan to hear (though hearing it live has a special "prohibition" attached to it according to him, so..).
These posts never seem to come out they way I would like them to...
Rav Ovadia Yosef (talking about his efforts against the "Ashkenazi Establishment" to allow weddings in the Israeli rabbinate during the "three weeks").