The market is offically open on Wednesday and Saturday, but a small subset of vendors are there every day of the week.  There are dozens of produce vendors on the north side of Mamo Street selling a variety of local produce and seafood.  They have common produce like pineapples and bananas and more unusual produce like Jaboticaba and Guava.  On the south side of Mamo Street the vendors have many hand crafted as well as commercially produce items including Hawaiian shirts, jewelery, and prints.  The entire market is covered using tarp tents so the occasional Hilo shower is not a problem.  The market north of Mamo Street is on a paved surface and the market on the south side of Mamo Street is on a gravel surface so flat shoes make walking this easier.  Plan to spend at least an hour, but one can easily spend two or three.