PR. changes when SERP visit your site, and SERP visit your site when you invite it or something big happen for your site or a page added or deleted from your site and you change your sitemap and ...
They better hurry up with the pagerank update... I hated seeing my PR4 drop to PR2 and all the other thousands of pages on the site drop to a PR0.
either way, i cannot wait! I am hoping to get at least one to two points increase. a lot of people rely on PR as a way of judging sites now days... that is if they are a techie person.
There is no one to give you the correct answer to that question here, it is better you ask Google, if you want to know the particular date for the PR update.
No one will know except Google, its best to keep working on improving your site and adding great content for your customers, this will help you become a credible site within Google which is the only thing that really matters.