It might sound obvious, but if you are going to undertake a big promotion of the community to an external audience, make sure you do it on your best days.
Clean up the community first.
Ensure most discussions have a response, initiate some topical questions for newcomers to join in.
Check the onboarding journeys for newcomers work effectively (and the formatting of emails looks great).
Plan for greeters to help newcomers.
Line up quality blog posts and new ideas from members shared in the community.
Do all the things that will make the community look great.
And if you can’t do these things, don’t do the big promotion.