There is a dispute over what caused the deaths.
A statement on Ethiopia's state broadcaster said the mourning is to "commemorate innocent citizens who lost their lives because of the violence instigated by anti-peace forces".
Prime Minister Hailemariam Desalegn earlier blamed rioters for the "mayhem" which led to a stampede.
Opposition activists say the panic was caused when security forces fired teargas and bullets into the large crowd which had gathered for a thanksgiving ceremony.
They also say that the death toll is far higher than the official figure of 52.
There has been months of tension in Ethiopia's Oromia region after a series of protests in which Oromo people have been complaining of political and economic marginalisation.