Hi, Frank ...
It depends upon the particular voyage. Generally around noon or one o'clock pm and I've never seen a Celebrity ship without lunch.
If i were in Barcelona again I'd get on the ship at the last possible instant. What a wonderful city !!! You will have plenty of time aboard and to mill around on a ship when you can enjoy one of Europe's most fascinating cities seems like a crime just so you can have a free lunch.
Let me recommend an excellent restaurant in the Gothic Quarter, 4 Cats. They almost had to drag me out of there last time. There are also some excellent restaurants on the water.
You can check out of the hotel, leave your luggage in a holding room then come back to collect it. If you ask nicely, you can also usually arrange a late check-out but it's more organized to pack up and have your luggage held.
I highly recommend the Lonely Planet guide to Barcelona which you can buy at home or at almost any of the kiosks on La Rambla.
Buen Viaje !!!