Today is Holoccaust Remembrance Day - the 27th Day of the Hebrew month of Nisan.
Contrary to some thought, the day was not chosen to correspond with the beginning of the Warsaw Uprising [Nisan 14]. There were some who suggested that as the date, but it would have coincided with Passover celebrations, and wouldn't have been appropriate. The 27th was chosen because it lay between Nisan 14 and Israeli Independence Day. Even though it was separated from Passover, many Orthodox Jews don't observe it because it still falls within the month of Nisan, when mourning is prohibited. [Source / Source]