The time to set the feeder out is by mid-April, hopefully several days before the first bird arrives in the spring. The
Spring 2007 Migration of Ruby-Throated Hummingbirds Map can show you when ruby-throated hummingbirds were spotted in your area in previous years.
Hummingbirds are highly territorial, so you may wish to put out more than one feeder. By late July, the birds begin their migration southward to their wintering grounds in southern Mexico, Panama, the Bahamas, Haiti, and the Dominican Republic, making use of another large flush of hummingbird flowers in bloom at this time.