The fact that you are taking a poppy wreath leads me to believe that you are British and not flying from the USA before taking your train, so no airport.
Yes, it is a very good idea to take some Euros in advance. The 24th is a Saturday and some banks are open, but other than restaurants and cafés that are open all day, or bakeries and some flower shops in the morning , everything is closed on Sundays. Both banks and shops are closed Monday a.m. You B and B will not ask for settlement till you leave as presumably you have had to pay a deposit, but you will need some ready cash for meals and odds and ends. You do not say where your grand-father's grave actually is, and if it is outside Arras, you may have to take a taxi to get there. Public transport to war graves is thin on the ground, even to a huge cemetery like Notre-Dame de Lorette, particularly at this time of the year.
At the moment in the UK the best rate of exchange is from a Post Office. Depending on where you live some have the currency over the counter, others get it for you and you can collect your Euros two days later. Banks charge for exchanging money but if you are a customer of Nationwide, they do not charge for taking money out from a cash machine on the continent or using your debit card for purchases in France.
You should enjoy Arras. Besides the awesome main squares, the town has undergone a renaissance in recent years and is a pleasant destination. There also are some nice restaurants that serve good food .
Grandfather's grave at N.D. de Lorette