The setting is in the Salinas Valley, in California near Soledad. It is a rich agricultural region, which contrasts with the poverty of the two migrant workers George and Lennie. It was also something of an oasis for farm-workers fleeing the devastated "dust bowl" areas in the great plains areas of the states (in contrast, the Salinas Valley is referred to as the "salad bowl." Hence, it has some of the qualities of a "promised land."
Also potentially significant is the name of the nearest town--Soledad. In Spanish, Soledad means "alone", "solitude" or "loneliness"; "soledades" is a "solitary place.