On the Jewish calendar, Yom HaShoah falls on the 27th of the Hebrew month of Nisan, which means the observance will begin at sundown on 26 Nisan.
In 1951, when the Knesset (Israeli Parliament) established the observance, it did not want the day to interfere with Shabbat by coming either immediately before or after Shabbat. Therefore, if 27 Nisan falls on a Friday, the day is observed on Thursday, 26 Nisan. If 27 Nisan is on a Sunday, where the observance would begin on Saturday evening at the end of Shabbat, it is moved forward to Monday, Nisan 28, with the beginning of the observance on Sunday, Nisan 27.
Outside Israel, there is less agreement about the wisdom of moving the observance of Yom HaShoah. The United States Holocaust Memorial Museum follows the Israeli custom, as do many other Jewish institutions. The Reform and Conservative movements have decided not to move the date. The conflict with Shabbat is considered less severe, and maintaining the consistency of the date of observance facilitates planning communal observances.
Note that Yom HaShoah is different than the United Nations' designated International Holocaust Remembrance Day, observed annually on January 27, the anniversary of the liberation of the Auschwitz death camp.
Visit our Yom HaShoah page to learn when the Reform Movement will observe Yom HaShoah this year.