The sign for the opening of Downton says that it will be open on the 6th and Edith mentions that Bertie Pelham will be "In London on the 11th". Cora then tells Edith to invite Bertie to Downton on his way back to Northumberland. Which she does.
However, Cora says they're opening the house that weekend...
But they're not. "That weekend" is, as per the 1925 calendar, the 13th and 14th - which is the weekend after the house has been opened.
So, was it just a mistake by Elizabeth McGovern in the pronunciation or something entirely different?