You already paid the subscription costs for the second month Weekend Vrij. This is a refund of those subscription costs. It’s a little less than the amount for one month, since you cancelled it a few days into the second month. The minus sign indicates that you will receive this amount rather than having to pay it.

That is, in the future I have to cancel the subscription clearly on the day?

If you want the subscription for only one month, and don't want to pay for the second month, then you should indeed cancel it right after one month. In your case, if you had cancelled it on the 17th, you would not have been charged for the second month. Now you will have been charged for one day of the second month.

Thank you!!! Now I understand everything!

You should also have an invoice dated March 17 (in Mijn NS, the date on the invoice itself would be March 18) for € 34,95 The auto debit (and credit) will take place in a few days.