When your all day events span two days, it means the time zone is different from the time zone used when you created the events.
If you are syncing with a PDA, verify the time zone and daylight time settings are the same (and the correct one for your location) on both computers.
If the events span two days only in March, verify the 2007 Time Zone update is installed. See Daylight Saving Time Updates for more information.
How to Fix
When the time zone change is permanent, use the Microsoft Office Outlook Tool: Time Zone Data Update Tool for Microsoft Office Outlook to move the appointments.
Alternately, you can set the computer so the times are correct, export the calendar to Excel or CSV format. Once exported, correct the clock so the times are correct then delete the appointments from the calendar and import the file.