I had the problem with a multi-day event not showing up in the weekly view. It started on a Sunday (week starts on Monday) and was only shown in the following week. I couldn't find another solution but changing the code of the template file calendar/theme/calendar-week.tpl.php:
In line 67 the for loop goes from 0 to 5:
for($j = 0; $j < 6; $j++)
but it should rather go up to 6:
for($j = 0; $j <= 6; $j++)
Or am I missing something here?