Its valid until you reach the border. In Belgium you need an other ticket.

You can buy a ticket @ NS international or a ticket machine/office and select a travel discount of 100% in the Netherlands. You'll only pay for the Belgium part of your journey.

Weekendvrij is valid up to the Dutch border. You do need to pay for the Belgian part.

Buy an international ticket at NSInternational and fill in a reduction of 100 percent for the Netherlands. For Belgium you need to pay the full price. Open the gate with this ticket and take your Weekendvrijcard with you as a proof for the right of 100 percent reduction. It is quite a difference in price.

Edit: There is no need to ask it twice. I just noticed you already got a correct answer of Slidestops below.

BUT if you are ju/senior its most often a lot cheaper to break it in 3: NL-deel, via whatever to Noorderkempen ONLY (=6€/single) and via b-rail the rest: youth Go1 for about 7,50/single or Senr for 7,20 dayreturn. For anyone-if you come back same weekend- weekendreturn (=in BE just a little more expens. as 1 single).

IF you do this more often-book at least 1 week before and the price falls a lot.

I've deleted the duplicate topic and moved Slidestops response to this one. Asking twice is indeed not necessary. :)