It depends on your location, but it has to be after all risk of frost in the Spring. Even a light frost can kill squash plants. You can often plant the seeds outside in the garden at the same time as you would have planted started seedlings and you still get squash around the same time. This works best with a warm dry spring.
As a bonus, sometimes squash seeds will grow after squash has been discarded in the compost, producing volunteer plants even with -25 Celsius overwinter!