My wedding was held in a hotel on a Sunday. It costs a lot more to book a registrar for a Sunday, so I can understand why changing the day might make the whole thing cost a lot more.
We got married very quickly too, less than four months from getting engaged to getting married. And it was only eight months from our first meeting to our wedding.
So we had to send the invites out very quickly to make sure people had as much notice as possible.
And when MIL saw the date on the invites she rang everyone on DH's side of the family to tell them we had made a mistake and that we obviously meant to write the day before on them because "nobody gets married on a Sunday." While she was at it she also told them I was pregnant and that's why it was a shotgun wedding at short notice (I wasn't, cheeky cow, just because she had a shotgun wedding.)
It was a massive mess trying to contact everyone when we found out, because of course we were the last people she rang to tell about the 'mistake' with the date.
And her and PIL went to the hotel on the pretext that they were meeting us (they weren't, we didn't know they were going) to have a nose about and make a few demands requests on our behalf.
Don't change you day. It's your mums fault if she doesn't come and you will still have a lovely day. Does the war veteran have a wife who could come and have her hair done in your mums place and be a witness?