Still waiting - Bilbao to Zaragoza train tickets for May 18th - should I give up and use bus?

Try using https://www.thetrainline.com I found some tickets for the days you mention. If you don't see them in the immediate times listed, click "later" to show more times for that day.

Odd.. I'm looking at May 14 and there is one at 15:20 that goes from Bilbao to Zaragoza with Alvia from 18,00 €, does not seem sold out to me, but from May 15-19 tickets are not available (not necessarily sold out), there is availability again starting May 20.

https://venta.renfe.com/vol/buscarTrenEnlaces.do?c=_kAb4

Have you considered our excellent buses in Spain? Like ALSA? I know many Americans are hesitant to use cross country buses but the ones in Spain are like nothing in the USA. If you want to travel on May 18 I'd strongly consider ALSA.

@Carlos - thank you - you are right. I can buy a ticket for the 14th. We're not against using the bus - just like to keep our luggage with us and feel like it's easier to use the bathroom on a train. But I'm certainly ready to buy bus tickets in the next two weeks, if the train options don't show up. It's just a weird chunk of time to have no tickets.

Omio shows tickets not for sale for May 16, 17, 18 and 19. It does show tickets available for May 20.
It is similar on both The Trainline, Petrabax and Renfe websites.
I suspect there’s work on the tracks scheduled for that route during those dates.

We traveled between Bilbao and Zaragoza last summer and ran into the same problem. Not sure why… We just went by Alsa (bus) instead.