Readership peaks midweek, midday (US). But that doesn't mean it's the best time to submit a story, because there's also more competition. On a weekend, 2 points will keep you on the frontpage for a significant time. On a weekday it might not get you on at all.
From what I've seen, it only takes 1 upvote to hit the front page, and stories (if they aren't killed) tend to stick around for at least 24 hours. My hypothesis is that there isn't a best time.
This might not be the best barometer for submitting a story, but take a peak at http://www.hackrtrackr.com/, I was surprised (still am) how many people participate from within middle[skewed easterly] america. I've always operated under the assumption that HN activity scales according to eastern standard time.
it could go either way but in general x seconds after reading it (YMMV) and judging it HN worthy. i have found submitting stories prior to reading them, or even knowing that they exist, to be an inferior method.