The first Holocaust Remembrance Day took place on December 28th, 1949, on a day designated by Israel. Because Israel wanted the holiday on a particular day of the Jewish calendar (so many days to mourn, how to choose?) The problem with choosing a day on the Jewish calendar – the exact date could vary from year to year for those not observing the Jewish calendar, similar to how the date for the Christian Easter varies from year to year, based on its relation to the Jewish Passover.
In November 1978 Jimmy Carter signed an executive order making April 28 and 29 official “Days of Remembrance of Victims of the Holocaust.” The date was chosen as the date in which U.S. troops liberated the Dachau Concentration Camp in 1945. The first of these days of remembrance was held in 1979 in a ceremony at the Capitol Rotunda, led by Carter.
Sources: haaretz.com (Israeli news), ushmm.org,