As promised, here's part two (or is it part three?) about Tachanun, that most glorious of Jewish penitential prayers. As always, remember to consult your local orthodox ehrlicher rav before doing anything based on anything written by anyone anywhere.
Halacha 5: The custom is not to say tachanun in the house of a chosson (groom), on the day he enters the chuppah (gets married), since it's a yom tov for him. Tachanun is also not said in shul on the day of a bris mila, since the Jews accepted the commandment of bris mila with joy. We also do not say tachanun when a chosson is present in shul, and also not in the house of a mourner, since the seven days of mourning are compared to the seven days of rejoicing with regards to forbidden activities, as the verse states (Amos 8, 10), "And I will turn over their rejoicing to mourning," and all this also applies by the extra penitential prayers said on Monday and Thursday.
Even after one leaves the house of a chosson or mourner and comes to his own house tachanun need not be said, since he's right now holding immediately after Shemone Esrei (evidently he left the house right after the repetition of the amidah), and since he skipped tachanun in its proper place (immediately following Shemone Esrei) he has skipped it entirely (i.e. once a person doesn't say tachanun, he doesn't have to go back and say it once the condition preventing him from sayng tachanun departs), and this is also the law for the extra penitential prayers said on Mondays and Thursdays, which are, according to the custom in our country (Lithuania) every single day, said before nefilas apayim, even though they are obligated to be said, as explained in Siman 134 (unlike nefilas apayim, which is a custom, as explained in the previous post on tachanun).
Nevertheless, since the obligation is only by custom, and nowadays we don't say them in the time of their being obligated (every single day), when they are said immediately following the amidah before nefilas apayim, therefore once they are prevented from being said (as explained above) they need not be made up later in the day.
On Rosh Chodesh, where the custom is not to say hallel in the house of a mourner, every person should say hallel by himself when he comes to his house, since hallel is a mivtza the entire day, and its not mandatory to say it immediately following the amidah, as we see that sometimes it's obligatory to say it before the amidah (Siman 122), and it need not be said immediately following the amidah since its time is the whole day, even for a single person davening on Rosh Chodesh. The only reason we say it immediately following the amidah under normal circumstances is because of the general rule of "The diligent perform mitzvos at the first available opportunity." Another reason is that hallel is not comparable to the extra penitential prayers said on Mondays and Thursdays, since once those prayer's obligation is nullified (because the person was in the house of a mourner, and a mourner need not say them because he's considered to be in a festival [with regards to forbidden activities]). Hallel is different, however, because the reason it's not said is because of the mourning present in the house, and we don't want to say (Psalms 116, 17), "The dead do not praise you, Hashem," and it would appear as if we're making fun of the dead. Therefore it's necessary to leave the house and recite hallel, just as we find by someone who is making a journey and comes upon graves, that immediately after he finishes the amidah he must distance himself and recite the hallel, so that it doesn't appear that he's making fun of the dead.
When does this apply? By the hallel of Rosh Chodesh, which is only obligated by custom. The only time we say hallel in a mourner's house is on Chanuka, which is an obligation from the Rabbis, and a mourner is also obligated to say it. The custom is, however, that a mourner within thirty days, or if his parents passed away then within twelve months, doesn't act as chazzan for hallel because the congregation is then happy like on Shabbos and the festivals.
Halacha 6: We don't say tachanun on the day of a bris mila when davening in the shul where the bris mila will take place, but in another shul (in town) we do say it. In the winter, when circumcisions take place in the home, the custom is not to say tachanun in the shul where the person making the bris is davening. All this is talking about shacharis, before the bris happens, but by mincha, even if the baby is present, we say tachanun.
On the other hand, tachanun is not said when praying by a chosson the whole day, or if one comes to daven in a shul on the day when the chosson goes under the chuppah (to get married). There are places where the custom is for grooms to not attend shul on the day (or a couple days before) they get married so that the congregation is not prevented from saying tachanun. If the chosson does go to shul then tachanun is said, except on the day of the wedding.
There are those who have the custom to not say tachanun the whole seven days when the chosson is in shul, because all seven days are for him like a festival. On the eighth day tachanun is also not said by shacharis because the chuppah happened close to the evening, and seven full days are not completed until the evening of the eighth day.
There are places which have the custom to also not say tachanun by mincha when davening with the circumscised baby. However, in the shul we do say tachanun, even though the person who made the bris is present. On shabbos we say Tzidkascha Tzedek even if the seudas mitzva is only made that night.