It's seems a bug in Piwik archiving algorithm. In screenshot #1 that I show there are 0 visitors in october 19. By the way, actually I had 95 visitors. See screenshot #2: Little dig in database reveals that archiving was made on 16:59:59 OMST (that is, 9:59:59 UTC). My server works in OMST timezone.
When I tried to change field
ts_archived in mysql table
archive_numeric_2011_10 to value
2011-10-19 17:00:00, the Piwik began to show visitors count correctly.
I don't know from what point of source code this issue come from. Developers please fix it :)
Thanks for the report and the info. What timezone did you set the website to in Piwik > Settings > Websites ?
Note: we also did some work on the same issue for Piwik 1.6, see ticket #2548
See another report in: http://forum.piwik.org/read.php?2,82670
(In ) Fixes #2728
I couldn't replicate the issue since it's an edge case, but I have the feeling that this would fix it (since the bug report was quite precise and highlighted a 1 second offset problem)