Tachanun is a prayer that is said on regular weekdays, and is a prayer of confession for our wrongdoings. It is customary to beat our chests when we say this prayer.
On Yom Kippur we say this prayer in a more detailed version, since it is a special day of returning to G-d in sincere repentance, and gaining forgiveness. Nevertheless, the short version is said on regular weekdays.
On festivals and Shabbat we skip this prayer, because on those days we emphasize our relationship with G-d through our love for Him, rather than focusing on fixing what we are lacking.
During the whole month of Nissan we do not say Tachanun, since the majority of its days are days of joy. The first twelve days correspond to sacrifices that were brought by the heads of each of the twelve tribes when the tabernacle was dedicated, then of course there are the days of Pesach, so that the whole month is a special month of miracles and redemption.
This is an appropriate time to provide you with a couple of pointers about Pesach:
1. It is important to eat Matzah Shmura at the Seder. That means matzah made from wheat that was guarded from coming into contact with water from the time it was reaped, in order not to become chametz.
2. Any chametz you own must be sold through an Orthodox rabbi to a non-Jew before Pesach. It does not have to be removed from your home or office, just covered and marked as sold. The rabbi buys it back from the non-Jew immediately after Pesach.
We suggest you discuss all the above with your rabbi — there’s no need to be embarassed!
Of course, you may turn to us any time …
Good luck!
Have a Kosher and Happy Pesach!