I think for the first big day, we'd want to coordinate the emails with press releases, lobbyist/govt relations efforts, etc. It could generate a lot of press and enhance the impetus to act. Journalists would ask the representatives what they're doing about it, giving it life outside our echo chambers.
And then, YES! I don't imagine it would be one annual effort; individual museums or smaller collaborative efforts of several museums (in a region, for example) could follow up multiple times. I mean, museums should have online advocacy *programs*--I'm not talking about a one off, I'm talking about permanent adoption of a tried and true strategy that every kind of nonprofit EXCEPT culturals has been using successfully for years. And Museums Monday is the way to kick it off.
Please, please, if you're interested, sign up at actforarts.org and we can start building a community around this. I really don't have enough free time to email you too much, I promise.