Certain appointments that are scheduled to occur on Saturdays (including December 31, 2011 and later Saturdays) disappear from Calendar in Outlook 2010. This behavior occurs after you install the update that is described in Microsoft Knowledge Base (KB) article 2633952. Specifically, the missing appointments use the Samoa time zone and that were created before you installed the update.
Note The update that is described in KB article 2633952 changes Samoa Standard Time from UTC -11:00 to UTC +13.
To work around this issue, run the Outlook Time Zone Data Update Tool that is available in the following KB article:
931667 How to address time zone changes by using the Time Zone Data Update Tool for Microsoft Office Outlook
To download the Outlook Time Zone Data Update Tool directly, click the following link as appropriate for your system:
Note This issue does not occur in Outlook 2003 or in Outlook 2007. Even though the appointment disappears from Calendar, there is no data loss, and you can still view the appointment. To do this, click View, click Change View, and then click List View. This issue does not affect meeting recurrences.
Microsoft has confirmed that this is a problem in the Microsoft products that are listed in the "Applies to" section.