The purpose of a reminder email is usually to remind the person who receives it of an upcoming appointment or an upcoming meeting. Whether it is written in a formal style or not depends on what you are reminding the person of. What is most important is that you send the reminder email at least a day or two ahead of the event and that the information about the date and time is accurate.

I think some honorific would help. People like polite manner. Not being rude is proper, I think ,