If you already have an NS Flex subscription, you can even start the subscription an hour before the start of your journey.
But please keep in mind that you can change your subscription only once a month. If you have changed your NS Flex subscription, you have to wait one month before you can change your subscription again (except when you have changed your subscription to NS Flex Basic, then you can change your subscription the next day).
Besides that, so now and then there are some technical issues in the “Mijn NS” environment, which causes that you can’t change your subscription. So, it could be useful to enable the subscription one day in advance, to make sure your subscription is active when you need it.