Those like myself in a perpetual rush to get out the door to work in the morning will enjoy a few moments saved by not having to say tachanun the entire month of Nissan. Why is it that tachanun, tzikascha tzedek, ta’anisim are all cancelled for the entire month? Some poskim make the following calculation: close to two weeks of the month would be exempt from tachanun because of Yom Tov, Chol haMoed, Isru Chag, Shabbos; the inauguration of the Mishkan took place at the beginning of Nissan, so those are days of celebration as well; since the majority of the month is exempt from tachanun, we just knock off tachanun from the end of the month as well.
The Aruch haShulchan, however, offers a different rationale. He writes that the zodiac sign for the month is Aries, the ram, which was worshipped by the Egyptians as a diety. The act of offering a korban Pesach from the very animal worshipped by the Egyptians demonstrated the falseness of their entire belief system. The entire month was thereby transformed from a celebration of idolatry to a celebration of Hashem’s dominion and geulah.
The nafka minah between these two approaches would be whether to skip tachanun during the last days of Tishrei, post- Sukkos. If the reason for skipping tachanun for the entire month is because we already omit tachanun for most of the month anyway, the same logic should apply in Tishrei, with its many Yamim Tovim. But if the reason for skipping tachanun depends on the unique character of Nissan, the same would not apply to any other time period.
As an aside, I’m just wondering about the association of Aries with Egyptian worship. The zodiac signs are based on ancient Greek tradition. Maybe someone with more knowledge of ancient Egypt than I have can shed light on whether these same signs carried significance in ancient Egyptian culture as well.