Farmers from around the Czech Republic will gather at the National Agricultural Museum in Prague to sell their products directly from their farms to the public.  A great market to go to shop for anyone who loves organic, unprocessed food.  From personal experience, the apples and other products bought at the Biojarmark are of top quality so I highly recommend it.  As with most events in the Czech Republic, there will be music and activities in addition to food and drink to enjoy at the market.