Launch days are a bad idea.
They build hype and anticipation. They shift the focus to the short-term at the expense of the long-term. They usually cost money and force the community manager to attract as many people to register for the platform in as short a time as possible.
Launch days are part of the big launch thinking. We published an entire (free) book to resist this thinking.
The alternative to launching is growing.
You don't target one big day, you have hundreds of small days. You have days when you invite a few more people to join the community, then a few more. You repeat this process without fanfare or a major launch.
You're in this for the long-term. You're not going to waste your potential and credibility on a launch-day blowout.
Almost every successful community we see today were grown, not launched.