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It's probably ideal to prepare the Google Docs several weeks in advance, so that somebody with a repeated announcement ("subscriptions are on sale") can add it all at once. Especially if you're doing that, it's probably fine to figure that (say) the publicity coordinator will add "you should pick up flyers", the rounds coordinator will add "This week we're starting the waltz class", etc. if they want them.
Emailed announcements
There's a lot of information about this, so see Emailed Announcements.
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So what would go on the website is just the announcement about rounds. The website automatically updates the dance schedule but rounds are a little more specific. Usually there are also a few more announcements; this is a template. Notice that the announcement, which is sent out on Monday, includes information about the rounds happening the next day.
Spoken
We aim to keep spoken announcements short and useful so we can go back to dancing. Below are some thoughts on what is and is not worth including, but it was the topic of a heated discussion on tech-squares-discuss (https://mailman-mit-edu.ezproxy.canberra.edu.au/mailman/private/tech-squares-discuss/2015-July/thread.html), so take it with a grain of salt. At some point somebody should to work conclusions out of that discussion (and possibly run them by -discuss again...), but that hasn't happened yet.
Announcements read at the dance:
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