Three things. First, confirmation that the window works for your team. Second, access to your domain (DNS) account, or a person who can make a change there during the window. The DNS change is what brings you back online, so this is the one item that can delay a migration. Third, a complete list of the web addresses that point at your Rock, including subdomains. It also helps to know early if anything sits in front of your site today, such as Cloudflare, a CDN, or a firewall appliance.