Q. Why is Yom HaSho’ah on 27 Nisan?
A. The date was chosen by the Israeli Knesset for reasons that are unclear. To many people, other dates seem preferable, e.g. 10 Tevet, a fast day marking the destruction of the Temple; or – the more obvious choice – Tishah B’Av, the major day of commemoration.
The choice of 27 Nisan may be to mark the end of the Warsaw ghetto uprising, though the uprising continued for at least another week, or because a date before Yom Atzma’ut denotes the relationship of the Holocaust and the establishment of Israel.
The date in Nisan creates halachic problems for observant Jews who do not say Tachanun (supplicatory prayers) during that month.