Chiapas [1] has two seasons, rainy and dry, the former extending from May to October, the latter from November to April. There’s far more variation in average temperatures depending on region, altitude and season. The highlands around San Cristóbal [2] tend to have cool agreeable weather, with daytime temperatures ranging from the 50s to 70s, depending on the season, with crisp cold nights, occasionally dropping near freezing—be sure to pack a jacket and knit hat!
Highlanders consider the central depression, where Tuxtla [3] and Chiapa de Corzo [4] are located, to be exceedingly hot, though many foreign travelers appreciate the warm arid climate, especially when San Cristóbal is experiencing one of its occasional cold fronts. Hot humid conditions prevail around Palenque [5], and even more so along the Pacific coast and the northern Gulf plains.
Links:
[1] http://www.moon.com/destinations/chiapas
[2] http://www.moon.com/destinations/chiapas/san-cristobal-de-las-casas
[3] http://www.moon.com/destinations/chiapas/tuxtla-gutierrez
[4] http://www.moon.com/destinations/chiapas/tuxtla-gutierrez/chiapa-de-corzo
[5] http://www.moon.com/destinations/chiapas/palenque/palenque-town/palenque-archaeological-zone