In doubt of whether to say Tachnun or not

Question

If there is a doubt in a particular minyan whether tachnun should be said or not, what should be done?

For example: There is supposed to be a bris in the shul however the kehillah is not sure if it's taking place that day or not (for those that have the minhag to not say tachnun when a bris is taking place in the shul, even when the Father might not be there.)

As well as a side note to this question, on a day when tachnun is not said (Rosh Chodesh, Tu B'Av, Channukah, etc.) may someone say tachnun if they want to anyway or is it "ossur" (forbidden) for one to say tachnun on a day when it shouldn't be said.

Answer 1

This Young Israel publication says (summarised) that in cases of doubt or of differing views the poskim say to tendsto leniency with regard to tachanun. One should omit tachanun, rather than include it. Some suggest the reason based on the inclusion of the shlosh esrai midos. which should never be recited without proper kavanah. However, not all versions include the shlosh esrai midos, due in part to this very reason. Nonetheless, there are other reasons be lenient. It is anyway omitted on some days and is subject to the dictum 'better a small amount with kavanah, than a large amount without'.

Peninei Halachah from HaRav Eliezer Melamed says

The Tur section 131 writes in the name of Rav Natrunai Gaon that since Nefillat Apayim is voluntary, it is customary not to recite it in the house of a chatan (groom), as writes Shut HaRivash 412 in the name of Rav Sar Shalom Gaon. Rabbi Yitzchak Ibn Giatproves this with the story from Bava Metzia 59b about Imma Shalom, Rabbi Eliezer’s wife, who would not let Rabbi Eliezer say prayers of supplication after the Amidah so as not to cause harm to her brother, Rabban Gamliel. It is from here we learn that it is not an obligatory prayer. Other Rishonim and Acharonim have written this as well. Birkei Yosef131:13 and Sha’ar HaTzion 131:15 write that in any case of uncertainty, it is better not to recite Nefillat Apayim, since it is voluntary.

Neither source specifies the nature of the doubt.

Answer 2

No source, but I think that, unless there is good reason to assume it will be held there, the Shul says Tahanun.

What is "good reason to assume"?

• If a member of the immediate family (eg., father) is there with an entourage.
• A sudden influx of people who don't normally attend the Minyan.
• A community-recognized Mohel is there with a bag or with large bulges in his pockets.

I know my examples might seem kind of silly, but if there's absolutely nobody in the Shul who knows whether a Berith Millah is being held there that day, I think the assumption has to be that there is no Berith Millah. Otherwise we would never say Tahanun, because you never know who might decide to pop in at the last minute before the Gabbai locks up and say, "I need to make a Bris! Right. Now!!"