related judaism.stackexchange.com/q/17967/759

"there is no requirement" It's actually 100% prohibited to do so in mourning. "There are no liturgical changes at all from what is done on a 'regular' Shabbat." How do you know this? You don't even seem sure of it yourself in the rest of that paragraph. Why should we trust you?

"since Tachanun is not recited on Tish'a B'Av itself, and, although the fast itself is postponed, the calendar date, actually is Tish'a B'Av" we wouldn't say tachanun or tzidkatcha tzedek on erev Tish'a B'Av even on the 8th of Av.

@Daniel That's true, but you won't see that happen for a few years.

So? Your reason is still wrong. It's not because the date is the 9th of Av. It's because it is erev Tish'a B'Av

@Daniel Why is it wrong? There is no Tachanun on 9 Av. QED. Do you not say Tachanun at Mincha on the first day of a two day Rosh Chodesh only because it's Erev Rosh Chodesh?

@DoubleAA Well we say Av Harachamim on 9 Av so the no-tachanun day definitely seems to be the day of the holiday rather than the calendar date.

@Daniel It's both. Actually, the fact that it is Tish'a B'Av on the calendar would be a stronger reason, as we would omit Tzidkatcha based on any day where Tachanun would otherwise be omitted. In Jerusalem, for example, they omit Tachanun when Shushan Purim is on Shabbat even though the celebration is on Sunday.

@Daniel Or Av HaRachamim isn't perfectly tied to Tachanun (cf Shabbat Mevarchim in Sefirat HaOmer)

@DoubleAA What does Shabbat Mevarchim have to do with tachanun? AFAIK, Av Harachamim is omitted on all days when tachanun would be omitted. That rule is printed in most siddurim. Therefore if we say Av Harachamim, it must not be a no-Tachanun day. If you're saying that rule isn't always true, I'd ask that you back your claim up.

@DanF That could easily be because the celebration is on Sunday rather than because the Shabbos is the 15th of Adar.

@Daniel I'm giving you examples of days where despite their celebratory nature we say AH anyway. Why is that confusing?

@DoubleAA Because I can't see how it's related to our discussion. Shabbos Mevarchim (besides during the omer) is another time besides non-Tachanun days when we don't say Av Harachamim.

@Daniel I think you're taking your rules and day-categories a little too seriously. Some sad things we omit when happy in certain ways and some we don't and everything is a gray-scale.

@DoubleAA Maybe this question is unanswerable because if 9 Av weren't Shabbos, it would be the day of the commemoration of Tish'a B'Av. So we're trying to figure out what would happen w.r.t. tachanun if 9 Av were not Shabbos but Tisha B'Av was also nidche for some reason. I suggest that since my siddur says that we omit av harachamim on days when tachanun would be ommitted and since we don't omit av harachamim on 9 Av, tachanun would not be omitted on that day. Therefore the reason to not say tzidkatcha is just because the following day is the Tisha B'Av observance.

@Daniel I think that is a terrible approach to this question. We need to determine the nature of the reason to omit Tachanun on Tisha b'Av, decide if that applies to 9 Av OR if Shabbat 9 Av retains aspects of Tisha b'Av, and then determine if that reason (or whatever aspects of it apply to Shabbat 9 Av) applies to the various liturgical changes which are conceptually similar to the rules about Tachanun.

@Daniel The argument can be made for both reasons regarding Tzidkatcha. I'm just suggesting that my own thinking is that the calendar date itself is the overriding reason for controlling Tachanun omission, and, thus Tzidkatcha, not the postponement. At the end, it amounts to the same action, anyway. Av Harachamim has its own rules, apparently. I agree that most Siddurim state that it is omitted on days that Tachanun is omitted, which is what puzzled me. DoubleAA claims that it has separate rules, so I need to research that, further.

@doubleaa I agree. What I'm doing is #2. I'm saying whatever aspects of tisha b'av apply to shabbat 9 Av do not apply to liturgical changes conceptually similar to the rules for tachanun. I prove this by the fact that Av Harachamim is said.

@DoubleAA in other words, I'm insisting that if you want to decouple Av Harachamim from tachanun, I want to see a justification for it since many siddurim include that rule.

@Daniel That looks more like #3 than #2 and I don't know how you are attempting it without first doing the first two steps... You haven't even established which kinds of missing tachanuns can affect which other liturgical aspects and why!

@Daniel Consider kadisha.biz/showItem.aspx?levelId=59613