I am not really a busy guy, but I do write down important dates and appointments on a calendar. After being a long time Palm user I have migrated to Google Calendar to help me organised, with a little help from my Nokia E63 phone. Google Calendar can synchronise with the Symbian phone using Mail for Exchange application, so any scheduled appointment in Google Calendar will automatically appears in the phone. Recently Kontact, a personal information management (PIM) application suite for Linux, gained support for CalDAV syncing. So I can use Kontact as a Google Calendar client too.
So far so good. But there are some drawbacks. Online and digital calendars do not support Indonesian holidays by default, those red dates do not appear. You can subscribe to Indonesian holidays calendar provided by Google, but it seems unmaintained. Recently I find a good Indonesian holiday calendar on Mozilla site, made by Achmad Junanto. Of course it is meant be used with Mozilla Calendar applications, but you can also add it to Google Calendar, or any calendar application that supported ICS files such as Apple iCal or Kontact.
Sadly Mail for Exchange only sync one calendar, so I cannot see the holidays on the phone. I wonder whether this problem exists on other phones. Perfect PIM syncing seems still elusive in general, and I am still searching for good solution.