Why is Sam cooking such big meals when there are no customers?

When Max orders roast pork tenderloin with apple sauce and mashed potatoes, George tells him: "It isn't ready yet." Then Max orders chicken croquettes with green peas and cream sauce and mashed potatoes. George tells him: "That's the dinner." Evidently Sam is doing a lot of cooking back in the kitchen. Yet there are no customers except for Max and Al. Sam must be doing the cooking now just because there are no customers. The place is described as a "lunch room." Tomorrow Sam will be too busy preparing short orders and will have no time for such heavy cooking. He is obviously getting the food ready for tomorrow night's dinner. A few dinners may be served on the night of the killers' visit, but most of the pork tenderloin and chicken croquettes will be eaten on Monday. All Sam will have to do is warm it all up. It takes a lot of time to prepare the food for cooking and then time to cook. That is why he is getting it done tonight.
Obviously, this is a Sunday night. That explains why there are few customers. It also explains why Ole has been lying on his bed all day. He is a big man who must be a manual laborer. He does not show up at Henry's Lunch Room at six because he does not work on Sundays and does not acquire an appetite. The killers may have decided to ambush him on a Sunday night because they thought the little diner would be less crowded. They were right about that but wrong in believing Ole would be there at six o'clock any night of the week.
By inference we can assume Henry's is a popular place during the week. It will probably be packed for lunch on Monday afternoon and packed for dinner. Sam has to get his heavy cooking out of the way before he gets too busy. These dinners, featured with little inserts on the menu each day, used to be commonly called "Blue Plate Specials," and restaurants like Henry's were commonly called "one-arm joints," because everybody ate at the counter and leaned on one arm while shoveling in food with the other. Most such diners have been replaced by franchise fast-food operations like McDonald's, Burger King, Jack in the Box, Carl's Jr., and Wendy's.
Hemingway is being intentionally ironic when—less than an hour before they expect to kill Ole Andreson with a sawed-off shotgun and then kill George, Nick, and Sam with the same weapon—he has one of the insensitive killers order roast pork tenderloin with apple sauce and mashed potatoes and then chicken croquettes with cream sauce and mashed potatoes. The fact that they did plan to kill the three witnesses is proved by what the killers say after they give up waiting and are on their way out:
"So long, bright boy," he said to George. "You got a lot of luck."
"That's the truth," said Max. "You ought to play the races, bright boy."