The weather is almost always pleasant. In any given year, the area offers about 350 days of sun. The few wet days are likely to occur August-November, when high winds are also common. (If rainfall is especially heavy, the road between Cabo San Lucas and San Jose del Cabo sometimes floods out.)
The temperature averages 80 F/26 C year-round, but expect evenings November-April to be cool enough for a sweater. It can get cold in January and February, dropping into the 50s F/10-15 C at night. For some people, the water can be too cold for swimming at that time of year.
July-October is hottest, with temperatures reaching well into the 90s F/30s C. However, the dry heat makes the temperature bearable, and ocean breezes help. Nights cool down to a pleasant temperature during the hot time of year.