As for the days, Sunday followed by Saturday and a less extent Friday got most visitors.
As for the times, the afternoon to midnight UTC, got most activity.
The difference between the days is around 10%, the difference between day times goes from 50% to 100% (doubles between low to high).
That said, it doesn't really matter much when new uploads are posted, there are always plenty takers. Using low visitor times to post got the advantage that the site responses faster.