In the first two paragraphs, Dahl sets up the atmosphere as a warm and cozy home. Mary Maloney is clearly a devoted wife. The house is "warm and clean." This indicates that it is loving and that Mary works hard to make sure the house is clean and inviting for her husband. The curtains are drawn, indicating that it is evening with the sun peering in, but also indicating that the house is enclosed. That is, this is their own private, intimate world. She has everything set up for her husband and herself. A lamp is lit for each of them and drinks are already prepared with extra ice nearby.
The second paragraph also illustrates what a devoted wife she is. She glances at the clock repeatedly "without anxiety, merely to please herself with the thought that each minute gone by made it nearer the time when he would come." She is glowing with warm feelings for her husband but also with the glow of being pregnant. She seems completely satisfied with her life at this point. She has a "slow, smiling air" and this means that she is calmly and comfortably happy. Here, "air" means the quality of how she expresses herself. Her skin is "translucent," meaning that it is clear and glowing. Her eyes are "placid" which means that they are peaceful.
She is a devoted wife, with comfortable home, a seemingly solid marriage, and a child on the way. This opening atmosphere and tone of a loving and idyllic home and marriage will create a more dramatic shock when the reader learns of the events that follow.