Hello.
Welcome to the Three Community.
You can buy it now or just before if you like. Either way, the addon will remain on the account until you use the days. The days activate on their own when you use allowances abroad, so it's fairly automated once you have it.
For the last day there, you will only need to ensure that your spend limit is high enough to allow you to be charged for that one day where you don't already have a pass. Since you won't have an active pre purchased pass available on the last day, the system will automatically charge you for 1 on the day, this will be £2.
Pete.
