Not sure what calender you are using to build your reminders but using outlook as my basis. 1) The calender day starts at midnight or 0000 hours. If you have a tone reminder set for an all day event, then it will sound at the start of the day 0000 hours. Another example is if i uncheck the all day event, then it allows me to enter the times and I can say from 0800 hours (8:00am)-1700 hours (5:00pm)=the event, then the alarm will sound at 0800 hours.
Recommendation is that you unselect the verbal alarm for an all day event, it will then pop on the screen but not sound off, this is a function in outlook, do not know if that is available in enourage or iCal, I am sure it is though. Or if you need the verbal then you will have to uncheck the all day event and select times for the alarm to sound. Sorry to say, but the "calender" day starts at midnight.
Hope that helps,
Good Luck!