A tianguis is a weekly open-air market that sets up in the street or a lot and packs down the same day. The word is Nahuatl, the format predates the Spanish, and it is still where a large share of everyday shopping happens. Every town on the peninsula has at least one, usually on a fixed day of the week.
Expect produce at a fraction of supermarket prices, plus household goods, tools, clothes, pirated DVDs, plants, and a whole secondhand section — the ropa usada and hardware stalls are where expats find things that are otherwise a trip to the US. Sundays and Saturdays are the biggest days in most towns, but the day varies by neighborhood, so ask a neighbor rather than guessing.
Bring cash in small bills — most stalls do not take cards, and a MX$500 note early in the morning will be a problem. Bring your own bags, and go early for produce and late for discounts as vendors pack up.
Haggling is normal on secondhand goods, tools, and clothing, and not normal on food — produce prices are posted per kilo and are already low. Ask "¿Cuánto es lo menos?" on the negotiable stuff and leave it there; grinding a vendor over twenty pesos reads badly.
The food is the underrated part. Tianguis food stalls — birria, carnitas, tacos de canasta, fresh juice, tamales — are generally excellent and cheap. Pick the stall with the longest local line and the highest turnover.
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