Hi Alan,
I try to encourage people to capture their images during the week of the assignment since that provides an incentive to us all to constantly practice and improve our photography. However, I don't discourage folks from submitting images that were taken earlier. Even the process of examining an image to see how well it meets the assignment objectives is a good learning experience.
Short answer: It's OK to submit images that were taken earlier (but even better if you can take them during the week of the assignment).
Keith