I don't see Launch Day being a real big "throw the switch" Hollywood drama moment. (SE probably doesn't want any drama moments. ) The full version of the game would have been running since early access about 3 days before.

As for login numbers, only SE knows. Phase 4 should be the biggest ("come play FREE!!"), but at least it's "beta" and SE has leeway to smooth any rough patches. Then after a little downtime it's Early Access and perhaps the fewest logins because it's just pre-orders and legacy. "Play for free" time is finally over.

Then 3 days later we roll past "official start of service", probably without downtime, but v1 are back, plus any new launch day starts. There will be a lot of registration code registering that day. Like Dhalmel says, it may be the most stressful at the accounts customer service.