Flights at this price are available from the middle of January through the end of February, with returns available through early March. Departures from JFK are valid Sunday through Wednesday and returns from TNG are available on Monday through Fridays. Other travel days are just slightly more at about $480 roundtrip.
As an example, searching on Iberia' site, I found seats departing JFK on Tuesday, February 12 with return flights from TNG on Thursday, February 21, 2019. Availability is wide open at the moment, so you shouldn't have any problem finding dates that work with your schedule.
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Need a room in Tangier? Here's a list of available accommodation for the mentioned dates on Hotelwatchdog.