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 slighty earlier) Bioferia de Miraflores: Parque Reducto No. 2 Av. Benavides y Via Expresa. When I’m chasing the fresh organic 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.
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).