The market is only open at the weekends, Saturday and Sunday from 7am until late. Some of the stalls do open during the week but not sufficient to warrant a visit. One alternative to Chatuchak is the new Suan Lum Night Bazaar which is open daily read our review of Shopping in Bangkok for more information.
Getting to Chatuchak has become a lot easier since the Skytrain system opened and now you can simply take the Skytrain to the end terminal called Morchit which is exactly where the market is located.