Birdhouses can be installed any time of year. Nesting activity can start in late Winter and continue thru mid Summer. Hole sizes relates more to who you keep out of the house more than who will enter the house. Small birds that can enter a house with a 1 1/4" opening will also enter a house with a 1 9/16" opening. But bluebirds that like a 1 1/2" - 1 9/16" opening can't use smaller openings. If a birdhouse has an opening of more than 1 9/16", it may allow a predatory bird access to the nest. Perches will also allow predatory birds easier access to nests. You will notice that none of the birdhouses offered have perches.