Mexico has diverse climates. Mexico City, at 2,250m altitude, has spring-like weather year-round (15–25°C) with a rainy season June–September (afternoon showers, not all-day rain). Oaxaca is similar but slightly drier and hotter. Yucatan is hot and humid year-round, with hurricane season June–November.
Beach destinations: Cancun and Riviera Maya are best December–April (dry, 27–30°C). Pacific beaches (Puerto Vallarta, Sayulita) are best November–May. The Copper Canyon is best visited October–May. The Dia de los Muertos celebrations (late October–early November) are worth timing your visit around.