I assume the main goal is to get onto the front page rather than just maximize views on the /newest page. I am willing to bet that getting on the front page at the least active time is still far better than being on the new page at the busiest time.
If I understand correctly, the determining factor is not the time spent on 'new' page but the number of upvotes. During less busy hours submissions spend more time on the 'new' page but are seen by less people per unit of time and therefore get less upvotes per unit of time.
The time itself means nothing. It's about ratio of time on new page to upvotes per unit of time which might be higher during busy hours. That means that it's not impossible that the correlation between time spent on new page and probability of getting to main page is negative.
However, to get on to the front page all that is required is that your 'hotness' score (approximately votes/time) is better than most of the competing stories.
More time on 'newest' -> fewer stories posted. The 'hotness' requirement is lower. It's also likely that as no major press releases are made during the off-times that the competition is lower quality.
Less popular posts that might not make it during the week have a better chance of making it. Now, on a Sunday morning, the top three stories have about 20 upvotes each.
