The country's President Lech Kaczynski made the announcement as he visited the town of Kamien Pomorski, along with Prime Minister Donald Tusk.
The shelter's administration said that at least 77 homeless people were in the three-storey building, when the fire broke out late Sunday, killing 21, including six children, and injuring another 21 people.
People in the small Polish town are reported to be in shock following the fire, the worst in the country for almost 30 years.
Many people have laid flowers outside the ruins of the building and local residents have started a collection for the victims.
Emergency services fear the death toll could rise as 11 people are still unaccounted for.
A book of condolences will be opened in the presidential palace from 5:00 p.m. (15:00 GMT) until 7:00 p.m. local time (17:00 GMT).
Fifty five people died in 1980 when a fire swept through a psychiatric hospital in Gorna Grupa.
