More important than the time of day is the time of year - and it is not the best time of the year to use oxalic acid vapor. This is because most of the mites will be under brood caps where the oxalic acid won’t reach.
If you still want to do oxalic then you can do what @Dawn_SD has described and treat every 5-7 days until your accelerated mite drop after 24° is under 25 mites.