January sits in the northern half of the Mexican year. The Southern Ocean has gone quiet, and what the Pacific sends instead comes from the north: north-west swell out of the winter North Pacific, arriving on a coast that faces west.
Sayulita and San Pancho are what that means in practice. Both are lefts, both are consistent through the winter, and the Nayarit coast holds its shape while Oaxaca, nine hundred kilometres south and facing the wrong way, has almost nothing to work with. If your dates are January, your coast is already decided.
The water is the thing people get wrong. Nayarit runs a proper season: 25°C in January against 28°C at Puerto Escondido on the same day. Neither is cold, but a boardshorts assumption about the north coast in winter is how people end up buying a top on arrival.
