Hi Mancity1234, and welcome to La Tercia,
Maundy Thursday is a national holiday, therefore many shops and restaurants are closed. Dos Mares is open 365 days but I can't find any definitative information for Eroski opening hours (hopefully one of the residents can confirm?).
I am over with my family on the 29th March (until the 8th April); if somebody does confirm it is closed I am happy to pick up a few essentials the day before.
James