The weekend market is open on Saturday and Sunday from about 11am onwards till about 5pm.
We went there thinking that we could buy Icelandic crafts, etc., but we were disappointed.
It was basically a food market and some stalls selling what seem to be secondhand stuff.
After walking in the market for about 10 minutes we decided to go somewhere else.
Not really worth visiting. You can get most products there from the many grocery shops / shopping malls in Reykjavik.