Might this not be something that should show up in each and everyI concur with Brian. I was thinking that perhaps this calendar entry would be better placed on updates@, as that reaches far more users. Or perhaps there should be a separate list for site status.
I would hope that a mechanism exists for "the Grand Poo-Bah ofI wouldn't want that posted as a message in my groups, as a group owner I'd find that intrusive. However a status banner on the web pages (all of them), just below the logo bar, might make sense.
For an event like this I don't think an email blast to _all_ users is warranted: the vast majority are unlikely to visit the site during the affected hours anyway. A notice on the site pages would be better targeted.