Deployment of a new version of a site is a very tricky/risky procedure. At some point (i.e. until deployment is not completed) your service will work not as it is supposed to, which can result in reset session, HTTP errors displayed, etc. Whatever the result is it can really discourage the users of your service and, let’s be honest, will not look very professional (if not lame…).
That’s why at the time of deploying the changes, you probably should temporarily switch off the service and display the appropriate message so that the user knows the web site is being updated, and that is happening now.
Now, if you are hosting the service on IIS this is very simple to achieve. All you need to do is copy app_offline.htm to the root directory of your web site. Once the file is in place, it will be served to the user as a response to any request to your web site. Once you’re done with deployment, remove that file and the updated service will start working again.