As a large city Milan has of course a lot of markets of different types and sizes to offer for each day of the week, but mostly on Tuesdays and Saturdays.
Some of the most important of markets:
For fish you may want to visit one of the largest (10000 sq m) and most modern fish markets of Europe, the Mercato Ittico, It is open to the general public on Saterdays from 9.30-11.30 at Via Lombroso 95, It is halfway from city center east to Linate airport. Take Tram 16 till Via Varsavia Via Monte Velino. This market is where all the fish restaurants of Milan and surroundings get their fresh fish.
The fish market
The best overview on the Internet of the street markets in Milan is available at the Milano Weekend website