Good day, I charge 20 euros for a train trip on the weekend, but I have free trips on the weekends. Why do they take money out?

It sounds like a correction fee was applied, perhaps because you forgot to check in or out. You can contact customer service to check this and have it removed from your invoice.

Contacting customer service can be done by phone; they are available 24/7 at 030-751 51 55. You may also contact them by chat (click the link Chat with NS on this page) or by sending a message on X (formerly Twitter) or Facebook.

In the weekend, no corrections should be applied for a forgotten check in/out with Weekend Vrij.

The 20 euro is most likely for travelling 1st class, because a forgotten check-in would result in 50 euro and a forgotten check-out would be 12 euro (both not applicable in the weekend), 

If this was an unintended debit/creditcard check-in (OVpay charge), then see ovpay.nl for a way to get your money back.