These mountains offer the best midwinter trail hiking on the peninsula. Expect rain July through October. November and December are warm (in the low 70ºF's), lush, and pretty dry.
The road for the national park turns off from Mexico 1 at San Telmo and is one road that is paved all the way to the National Observatory at Km. 100. The National Park entrance is at Km. 84. Turn offs for Rancho Meling (Km. 50) and Baja Dark Skies (Km. 67) are along the way before reaching the park entrance. It takes about two hours to reach the observatory from Mexico 1.
There is no where to get gas, water, food, or any other provisions once you make the turn off from Mexico 1. Fuel up and buy any provisions in San Telmo before heading to the Sierra de San Pedro Mártir. Rancho Meling is the only place in the area to get a meal, but arrangements must be made with them in advance.
You should make a reservation in advance to stay at the cabins at the national park or at any of the surrounding ranches in this area. There’s no cell service or wifi service in this area.
Because of the elevation, this area stays refreshingly cool in the summer and can get very cold in the winters (average temperature below freezing) with a decent amount of snow. Even in summers, temperatures can dip below freezing at night, so be prepared if camping.
As a Mexican national park there is an entrance fee – currently about US $4.25 per person a day or buy an annual "passport" for under $30 that permits entrance to all the national parks of Mexico.
Being there in May it was a bit chilly with snow but sunny days. The town is small and nice, there are a few good places to eat and it's a base for starting the hike to Kazbeg and the viewpoint of Gergeti Trinity Church.