Foreign guests can stay in Belarus for 30 days without a visa, provided that they arrived in the country through the Minsk National Airport and through it will come back. This rule applies to citizens of 80 countries. An important nuance: the flight should not arrive from Russia or depart to the Russian airport on the way back, since there are no border controls in this direction.
More information on visa-free entry rules can be found on the website
of the Belarusian Foreign Ministry.