Working days in Belarus are Monday through Friday; Saturday, Sunday and public holidays are not counted. Foreign citizens of the countries who may now enter visa free may stay in the country for up to 30 days, which may not be extended. Note that the registration is done for you by your accommodation (hotel, hostel etc.) when you arrive and check in. You count your stay from the actual calendar day you arrive to the day you leave (and not each 24-hour period starting from the time on the day of arrival).
According to the Belarus Ministry of Foreign Affairs:
Information on visa-free travel via the checkpoint “Minsk National Airport”
According to the Presidential Decree, effective from July 27, 2018, foreign citizens of 74 countries can enter Belarus for up to 30 days and exit from the territory of Belarus only through the State border checkpoint of the Republic of Belarus Minsk National Airport.
The given order of visa-free movement through the airport does not extend to persons coming to Belarus by plane from the Russian Federation, as well as to those who intend to fly to the airports of the Russian Federation (such flights are considered as internal ones and do not have border control).
The term of visa-free stay is calculated in calendar days; the first day corresponds to the date of arrival, the last day coincides with the date of departure (regardless of the time of arrival and/or departure).
Visa-free regime does not extend to holders of diplomatic, service, special and other passports equivalent to them.
The trip is carried out for private, business, tourist or other purposes not related to employment, commercial activity and education, if the term of education exceeds 30 days.
To enter the Republic of Belarus in the visa-free regime via the checkpoint “Minsk National Airport”, foreign nationals should have the following documents:
a valid ordinary passport. The passport must be valid at least 90 days beyond intended date of departure from the Republic of Belarus;
financial means: amount equivalent to not less than 2 base rates for each day of stay or 50 base rates if the stay is full 30 days, in Belarusian rubles or foreign currency;
a medical insurance policy with coverage for at least 10 000 euros, valid in Belarus during the whole duration of stay.
For citizens of Vietnam, Haiti, Gambia, India, Lebanon, Namibia, Samoa a valid multiple visa of the EU member states or Schengen zone with a mark in passport on crossing the border of these states and tickets with confirmation of departure from “Minsk National Airport” within 30 days from the date of entry is a compulsory additional requirement.
NB: When staying in Belarus for more than 5 working days, foreign citizens need to register with the divisions on citizenship and migration of the territorial bodies of internal affairs. When staying in hotels, hostels, sanatoriums and other collective accommodation facilities the registration of foreign citizens is carried out by the administrations of such facilities by default upon check-in.
The term of visa-free stay cannot be extended beyond the established 30 days.