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Metered parking stops charging at 8 pm. Best bet is to park outside Zone 3 and walk a few blocks. If you're just coming to the milonga, park anywhere on the street.
On Saturdays, metered parking goes down to 50 cents an hour (2 hour max), so it's feasible to park very close and just feed the meter between classes. Enforcement ends at 5 pm for Zones 1 and 2, and isn't enforced for Zone 3.
Parking in the blue all day is the best bet.