Here they are called “bioferia” or “ecoferia” and as far as I can tell, there are only a few with a specific schedule. Most of them feature the same vendors, who keep move to different locations depending on the day.
Saturdays, 9-2 (maybe slightly earlier) Bioferia de Miraflores: Parque Franscico de Miranda (formerly in Reducto No. 2 Av. Benavides) y Via Expresa. When I’m chasing the fresh organics like from Vacas Felizes, this is where I go.
Sundays, 8:30-2, Surquillo, Mercado 1.BioFeria de Surquillo. Jr. Narciso de la Colina cdra. 5. Lateral al Mercado #1 Surquillo (Ricardo Palma y Av. Paseo de la República)
Sundays, 9-3, Ecoferia El Polo Green, Centro Comercial El Polo, Santiago De Surco
I’ve also heard about this one: Centro de Ventas de la Universidad Agraria
Av. La Molina s/n, La Molina, 8 a.m. a 12 m.
**** heard as of October 2018**** apparently there is a now a farmers’ market in Jockey Plaza in Surco.
And according to this website, there are others. Sadly the one in Miguel Dasso no longer exists (it’s now located in the Reducto as listed above).