If you are looking for a Formentera market, the island capital is where the week is centred: Sant Francesc de Formentera has three weekly markets, all within the town, all outdoors. Between them they cover every day from Monday to Saturday, so you rarely need to plan a whole day around a single opening.
Tuesday and Saturday are the days to aim for, with three markets trading. Monday, Wednesday, Thursday and Friday each have two open, and Sunday drops to one, so a Sunday visit is the one that needs checking in advance. The usual pattern is a late morning start, with most stalls working 10am to 2pm, which fits neatly around a beach afternoon or a ferry back to Ibiza.
Because everything sits in one town, the walk between markets is measured in minutes rather than bus rides. That makes Sant Francesc unusual among market towns in the Balearic Islands, where you would normally drive between one village market and the next. All three here are street markets in Sant Francesc de Formentera, set up in the open air rather than in a covered hall, so shade and a hat are worth having in high summer.