The photo was taken on December 18th. It can be afternoon, because the sunset was at around 3 pm. I suppose the tent was for maintenance (or guard)during St. Thomas Market, held in the city centre Esplanaadi Park.
This building was being renovated and the door was locked in October 2007 when I was in Zagreb. Is it a cafe now or a book store? Click the photo and see it in a larger size.