Most toddlers will be ready to move to one a day nap between 15-18 months. You will know it is time to move to one nap when your little one stops having their morning sleep, starts refusing their lunch sleep or is waking for extend periods overnight. All of these signs can also happen at the 12 month regression however 12 month is a little too early to go to one nap and if you stay consistent and keep putting them down for all of their naps it will all return to normal after a few days.
So once your little one is showing all the signs and you know it is time to go to one nap you may need to move the lunch nap forward to 11.30am and then gradually move it back to 12/12.30pm. You may also need to make time earlier also, starting a 6pm and gradually move it back to 6.30pm. By doing this you will help them to get use to one sleep and prevent them becoming overtired.
Dropping to one sleep will allow your little one to have a good sleep for 2-2.5 hours.