If your home page doesn't involve dynamic content (and even if it does, but you aren't touching those parts of it), keep your home page up.
If you aren't taking down your DB, keep the user sign-up up.
When someone tries to log-in, then display the message.
Keep your roll-out window to under 30 mins if at all possible (rolling out features in smaller batches makes this easier).
If your home page doesn't involve dynamic content (and even if it does, but you aren't touching those parts of it), keep your home page up.
If you aren't taking down your DB, keep the user sign-up up.
When someone tries to log-in, then display the message.
Keep your roll-out window to under 30 mins if at all possible (rolling out features in smaller batches makes this easier).