What’s interesting is that the idea of ‘the three weeks’, or ‘Bein Ha-metzarim’ as it’s known in the halakhic literature, was seen as a significant time period from a relatively early stage (I’d say either from late in the period of Chaza”l or the period of the Geonim), even though the customs were not nearly as developed as they are now and in fact differed in tone. Allow me to explain.
There is a significant body of evidence that the period was seen as a unit:
1) 17 Tammuz and 9 Av are linked by the mishna at the end of Taanit which record the 5 terrible things that happened on each. The mishna goes on to conclude by saying that we ‘diminish joy’ during Av.
2) The Midrash Eicha which applies the verse which mentions ‘bein ha-metzarim’ to the period between 17 Tammuz and 9 Av, and notes that it is a time which is particularly prone to bad things happening
3) The well known schedule of haftarot, which is first mentioned in a psikta (all of the Rishonim quote the Psikta) that I cannot locate, includes 12 sections – 3 of ‘puranuta’, 7 of ‘nechemta’, and 2 of ‘tiyuvta’. Tisha B’av and the Yamim Nora’im are the turning points – from puranuta to nechemta and from nechemta to tiyuvta. It was clearly sculpted around the Jewish year, and the 3 haftarot of puranuta would always be during the three weeks between 17 Tammuz and 9 Av (even if Rosh Chodesh Av falls out on Shabbat, according to the consensus of Rishonim).
4) There’s an odd Yerushalmi Taanit (4:5) which compares the 21 days between 17 Tammuz and 9 Av to the ‘ripening’ of an almond tree, based on Yirmiyahu’s vision (1:13-14) of a ‘almond rod=makeil shaked’, which happens to be from the first of the three puranutas.
The sense one gets is that the ‘3 weeks’ were indeed an acknowledged part of the year, but not as a time of mourning. Rather, it was a time during which bad things happened to Jews. ‘Puranuta’ is ‘misfortune’. The mishna is a laundry list of bad things that happened. ‘Diminishing joy’ during Av, the antipode to increasing it during Adar, takes the form in halakha of not doing business or adjudicating cases with non-Jews, because they w1uld have the upper hand during this inauspicious time.
The Shulchan Arukh states only 2 laws (unequivocally, that is; he also acknowledges that there are some who don’t eat meat all 3 weeks) that pertain to bein ha-metzarim: a) we don’t say ‘she-hecheyanu’; b) we don’t go out alone during certain hours of the day when the demon ‘Ketev Meriri’ is powerful (based on that Yerushalmi in Taanit). Here, too, we get the sense that the 3 weeks is an inauspicious time for the Jews (as the Arukh Hashulchan writes, the issue of saying she-hecheyanu is not one of sadness – indeed, there’s no prohibition against a mourner saying a she-hecheyanu. Rather, it is a bracha which acknowledges that a time is auspicious, when it is in fact not auspicious).
Even as recently as a century ago, the Arukh Hashulchan notes that the custom not to get married during the 3 weeks is because it’s an inauspicious time to get married. It’s a ‘siman ra’. Le-havdil, it’s like someone saying they wouldn’t get married on Friday the 13th (the Arukh Hashulchan actually permits getting married during the 3 weeks if need be. R’ Moshe Feinstein, half a century later, only permitted the night of the 17th o Tammuz, since he held the 3 weeks hadn’t started yet. R’ Soloveitchik didn’t even permit that).
At the same time, there has been a growing halakhic trend to see the 3 weeks as a time of aveilut. This has steadily replaced the perspective of ‘puranut’ as the dominant tone of the period. The extension of weddings to include other types of festive gatherings with music (‘rikudin u-mecholot’ = ‘dancing and playing instruments’, in the language of the Magen Avraham, who first mentions it in context of the 3 weeks), and the more recent extension of that to include any musical instrumentation, and the general stringency regarding shaving and haircuts during the three weeks, both of which parallel mourning customs, have taken hold primarily in the last few centuries but even more strongly and stringently in the last one.
Rav Soloveitchik’s relatively well-known chiddush about the ‘revese aveilut’ of the 3 weeks reinforced this conception. He says that the customs of the 3 weeks parallel the mourning customs of the 12-month mourning period, the 9 days parallel the customs of the 30-day mourning period, and on Tisha B’Av itself we ‘sit shiva’. This transforms the entire period into one of mourning – and this is a dominant theme of the Rav’s shiurim on the topic (admittedly, Tisha B’Av has always been linked to aveilut; the halakha goes so far as to say that one who is actually sitting shiva on Tisha B’Av can go to shul like everyone else, because we’re all mourners).
My point here is to acknowledge the trend. I find it quite fascinating to see that not only does halakha evolve – we know that – but that the meaning and observance of a particular ‘sacred time’ (I know, there’s no ‘kedushat ha-zman during the entire period; there is, however, an acknowledged time of year that is part of our collective consciousness) has been replaced over time (see what I wrote here about how we respond to the ‘Ship of Theseus’ dilemma). Why did this happen? Perhaps the sense of immanent danger during the three weeks gradually slipped away, and the need to observe a period of mourning, a temporal ‘zecher le-churban’ spot, took hold as a palpable sense of galut faded. Who knows? Does that even fit the time frames under discussion? Either way, it’s food for thought (which can even be eaten tomorrow) over the next few weeks.