If you double click the date e.g. '11 October' on the '1 day' per page setting on Calendar, you can enter birthdays etc as an event which will then appear immediately under the date, with no time specified, but you can set yourself a reminder whilst doing so. Therefore, you don't lose any space for appointments.
Alternatively, if you don't want to use the calendar at all, you could set a 'task' e.g.'buy granny a birthday card'with a date set to complete the task and a reminder date when to start the task.
There may be a better way, but I use the first method with Outlook 2000.