Both are neighbourhood events that welcome visitors rather than productions staged for them, and both are free to attend.
Wynwood Art Walk, the second Saturday. Galleries open late, the district fills, and the ticketed mural courtyard has historically opened free from about five to ten that evening. It is the one night a month when Wynwood's art and Wynwood's nightlife are the same event rather than two different ones. It is also, by some distance, the busiest the district gets, so arrive before eight if you want to see walls rather than shoulders.
Viernes Culturales, the last Friday. Part of Calle Ocho closes between 13th and 22nd avenues from five to ten for a free arts and music festival that has run for more than two decades, with local artists, live music and a free walking tour led by a city historian. Go hungry.
Both finish around ten, which is early for this city. Treat them as the first half of an evening: the Art Walk hands you straight into Wynwood's bars, and Viernes Culturales hands you into the live-music rooms further along Calle Ocho.
If your dates include a second Saturday, note that the same day is also when the downtown art museum is free to everyone. A free museum in the afternoon and a free art district in the evening is the best-value date on the Miami calendar, and it repeats every month.