Why did Mary decide to make supper in "Lamb to the Slaughter"?
In "Lamb to the Slaughter" by Roald Dahl, Mary originally assumes she and Patrick, her husband, will have a drink and relax at home before going out to supper, as that is what they normally do on Thursdays.
When Patrick gets home, though, he tells Mary something that upsets her a lot (it is never explicitly stated, but it is heavily implied that he tells her he wants a divorce). Mary doesn't seem to grasp this fully immediately, and insists on making supper for them at the house. She goes to the freezer and selects a leg of lamb. Patrick tells her he is going out for supper and does not want her to prepare anything for him, though, and Mary hits him in the head with the lamb, which kills him.
After she realizes what she has done, Mary decides to go to the store to get all the food she would need to make a full supper for she and Patrick. At the store, she tells the grocer that she needs to purchase vegetables because her husband surprised her by wanting to stay in for supper instead of go out that night, even though they usually always eat out on Thursdays. When Mary gets home, she proceeds to prepare the food, including the lamb. All of these actions help set it up so that Mary has an alibi when she calls the police about Patrick's death. The police believe Mary when she tells them she returned from the store to find Patrick dead, and even unknowingly help Mary get rid of the murder evidence by eating the lamb she cooked.