Is there certain times of certain days that it isn't applicable?
Do I have to mention the voucher when booking?
It says Advance Booking Required, does this mean that I can't walk in off the street some day I'm in town, and use my voucher?
There are no restrictions on the time of day, or the day of the week you can use the voucher. You will however have to reserve your table in advance (as with a normal booking at Koh) and there's no need to mention the voucher.