The 591-sq-km Reserva de la Biosfera Ría Celestún is home to a huge variety of animals and birdlife, including a large flamingo colony. You can see flamingos (via boat tours) year-round in Celestún, but they're usually out in full force from November to mid-March.
Morning is the best time of day, though from 5pm onward the birds tend to concentrate in one area after the day’s feeding, which can make for good viewing.