It's never too late or too early to put up nest boxes! Bluebirds and other cavity nesting birds typically begin nesting in March depending on where you live. But they usually have more than one brood per season and may switch to a new site for their second or third brood.
Or if a birds' first nesting is unsuccessful, perhaps due to predators, they may appreciate a better nest box.
So, you can put houses up year round. Some birds will even use nest boxes as roosting sites in the winter.