One we already missed, it was about an hour or two ago at 6am EDT. But the next one is later this afternoon at 4pm EDT/1pm PDT.
At 4pm EDT today, you have your shot to ask John and other Googlers webmaster and SEO related questions live.
I typically try to jump into some of these hangouts but I won't be able to make this one.
For more on these hangouts, see this page.