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The sources I use are my own access logs, of course, WebmasterWorld, and sometimes logs that you find that are open for browsing that you find from searching for Googlebot, slurp, etc.
Brett's Google Update History [webmasterworld.com] is the definitive resource. You may also find lazzerzubb's Google Update FAQ [webmasterworld.com] and Marcia's Google Everflux [webmasterworld.com] thread useful.
Google don't have a fixed schedule for the updates, but the Update history should give you a good idea.
i've heard that google is supposed to spider tonight/tomorrow, which is in line with the timeframes listed in the Google Update History. we'll soon see if the rumors are true!
You appear to be confusing spidering (also known as crawling) with the update, spidering happens all month, but the main spidering of a site is early in the month, and the update is generally at the end of the month using the data from the crawl at the beginning of the month.
