Tlacolula is the largest town in the Eastern arm of the Oaxaca valley. Market day in this town is on Sundays and on this one day of the week hordes of people come to town from nearby villages to buy, sell, and socialize. You'll find all kinds of goods for sale, from produce to live poultry to electronics. Block after city block is closed to traffic and vendors occupy both sides of the street, while shoppers and ambulant vendors fill the space between them. Tarps suspended by ropes overhead provide shade, but you will need to watch where you're going, and duck occasionally to avoid walking into a tarp or rope.
There are some specialties you should be sure to check out in the Tlacolula market. The food stands selling "barbacoa" are very popular here. This meat - either lamb or goat - is cooked in an underground pit and you buy it by weight. The bread made in Tlacolula is very good; there are a few types that are particular to this area, but one that you should definitely try is the "pan de cazuela." It has a swirl of chocolate and raisins in it; when it's really fresh it's absolutely delectable.