Personally, I would keep the "base" of the page the same and have a <section> [or <aside>] for "upcoming events" or things of the same nature that change often
kenroar - I saw your post in the Updates and SERP Changes thread about your steady drop, looked at your earlier posting history, and came to this thread.
I think what JD_Toims is saying is very important advice. I think that announcements should be asides or subsections, but they should not be the main focus of your page. Changing the page focus every month is a big confusion to Google, which is only a machine. If your site is about widget education, you really want to have a
consistent body of content on your page that describes widget education, in a consistent and relevant vocabulary... not keyword stuffed, but at least containing the vocabulary you're targeting in the same way from month to month.
I've always insisted that clients put announcements in a fairly small section in the right hand column. With html5, you can also go to an <aside>. Don't put announcements up at the top, though, unless you expect a lot of current linking to the topic.
Perhaps a blog or news site, driven by fresh and ever changing-content can change its home page content and expect inbound traffic on that topic... but, unless your site is a blog, and topical content is what's most important, I don't think that constant change is what you should be doing.
As JD_Toims suggests, you need to keep the "base" of your page the same, and the base of a page is not the bottom of the page; it's the introduction.
I have been changing the <h1> tags on that page for about a year now and everything was the same until this past Friday when I saw a 35% drop in traffic. I haven't recovered since that point so I was trying to narrow down the problem.
I can't say why you are changing, except to say that I'm sensing that what's happened recently is that sites which were ranking because of inbound links are, for whatever reason, no longer ranking for them... which might be indicating that those links are counting for less because they've been devalued... or it may be that Google is weighting or parsing either inbound links or onpage content (or both in combination) differently.
In any event, it's not likely just the h1 tag... Again, if you're changing that, it's likely that you're changing the introductory material on your homepage. Perhaps Google has changed how it looks at fresh content. Think more about your content, and less simply about the h1.
I'd also evaluate all key Panda site performance factors (things like loading speed), and take a quick look at your backlinks... just in case your site has been slipping or Google has been moving the bar.