The best time to go to Loreto, Mexico is October. Temperatures start dropping to the low 80s and the weather is not as humid as during the summer.
Prices for accommodations are also relatively low compared to Winter, the peak season.
In October, expect to pay around $100/night for a hotel room in Loreto.
Winter is very popular here, the climate is temperate, relatively dry and full of sunshine. If you really don't like it when it's crowded with tourists, avoid the holiday season, from mid December through early January.