Milly writes to her parents from Mullingar that it was "Fair day" yesterday—the day on which cattle were sold in provincial Irish towns. Farmers from the surrounding countryside would drive cattle they wanted to sell to the town. The market day was a festive occasion, often culminating in drinking.
Milly's letter implies that the presence of money in farmers' pockets was good not only for the pubs, but for other local businesses as well: "We did great biz yesterday." She seems to have picked up some of her father's appreciation of making money.