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. 

Good morning mikakikuchi and welcome to the NS Community!

The response given by Isitimela is correct, assuming you already have an OV-chipcard with an active NS Flex subscription. However, there are two other scenario's that need to be mentioned.

First off, if you have a personal OV-chipcard but no active subscription. You'll have to purchase a new NS Flex subscription via our website. This can also start on the day that you're ordering it, but you'll have to activate the product on the card at a ticket machine before you can use it.

Then, if you have no personal OV-chipcard yet. You need a personal OV-chipcard in order to use a discount product. You can purchase a new OV-chipcard in combination with a new subscription, but the subscription can only start 7 days after the date that you order, related to the official delivery time for an OV-chipcard.