The internationally recognized date comes from the Hebrew calendar and corresponds to the 27th day of Nisan on that calendar. It marks the anniversary of the Warsaw ghetto uprising.
In Hebrew, Holocaust Remembrance Day is called Yom Hashoah. When the actual date of Yom Hashoah falls on a Friday, the state of Israel observes Yom Hashoah on the preceding Thursday. When it falls on a Sunday, Yom Hashoah is observed on the following Monday.