Hi
I'm not sure if you have got your dates mixed up - Easter Sunday is the 12th of April - not the 19th..?
Either way, you'll find that East Monday isn't a Bank Holiday in Spain - they actually have the Thursday (Maunday Thurs) and Good Fri as Holidays. I think most shops will be open as usual on the Saturday, but closed on Easter Sunday (but then pretty much everything is closed on Sundays anyway, so Easter won't make much difference there).
The PW supermarket on site (J4) is usually open for limited hours (09:30 - 13:00 I think) on bank holidays :- so I'd guess it might be open Easter Sunday morning. Probably safer to stock up on the Saturday anyway...
Cheers
Russ