The Day of the Russian and Belarusian Peoples' Unity is being celebrated to mark the day in April 1996 that Russia and Belarus officially formed a "union state."
Officials said Belarusian President Alyaksandr Lukashenka has sent telegrams to Russia's top leadership to congratulate them.
Russian and Belarusian actors, musicians and dancers were expected to take part in an event to mark the day Monday in the Belarusian capital, Minsk.
The union state, agreed during the presidencies of Lukashenka and Russia's Boris Yeltsin, resulted in several pacts aimed at creating greater social, political, and economic integration between the two countries.
However, plans for a single currency and other measures have not yet been realized.
With reporting by ITAR-TASS