No, they don't announce them in advance. But there's minor Google algorithm tweaks every week, and Panda reruns every month or so.
@Google often tweets updates, but not always.
I usually know about a Panda update within an hour or two by following Searchengineland here:
SEL often has insider details on what Google algorithm tweaks are targeting, so it's a good resource to help you understand what to do and what to avoid.