This is the date established in 1951 by the state of Israel for the remembrance of victims of the Holocaust.
It falls on the 27th Nisan, which is the anniversary of the uprising in the Warsaw Ghetto in 1943. It comes a few days after the end of Pesach.
Ceremonies and services of memorial are held, often in large public places.
The Holocaust affected all Jews, regardless of their ‘denomination’. As such, it is observed in similar fashion by all Jews.