Arica is famously the city of eternal spring. Daytime temperatures sit between 19°C and 26°C year-round, the sun shines most days, and serious rain is exceptionally rare — it has been known to go decades without measurable rainfall. There is no bad time of year to come.
December to March is the southern summer and the busiest period — Chilean families on holiday fill the beaches and the buses run full. Surfing is best in winter (May to September), when southerly swells reach the breaks at Las Machas and El Gringo. The Carnaval Andino con la Fuerza del Sol, in late January or early February, is one of the largest folkloric carnivals in Chile — Aymara dance groups from the surrounding altiplano process through the city for three days.
If you plan to combine Arica with the altiplano (Parque Lauca, Lago Chungará), avoid the wet altiplano summer (January-March) when the high country gets short violent thunderstorms. April-November is the dry season for the highlands.