Hallo aineol,
it really looks, that a train is on that special day not the best way to get to Astorga. I also didn´t find a train or a bus directly from Bilbao to Astorga, but you can go with a bus step by step Bilbao-Oviedo-Leon and from there Astorga.
www. alsa.es
Bilbao Estacion Termibus 12:05
Oviedo 15:35
price: 41,84 €
Oviedo 16:45
Leon 19:00
price: 8,96€
Because of Good Friday it looks like it could be the end of the trip for that day.
I train would need 30 minutes to Astorga, so a taxi would be not soooo expensive

, if it´s necessary to arrive in Astorga on Friday.
Or at Saturday
Leon 10:15
Astorga 10:50
price: 3,90 €
There could be a local bus company too, which could manage it on Friday, but I didn´t find something in the internet.
All the best for you
Michael