Day in baby sign language looks like the sun rising over the horizon. Your non-dominant hand and arm should lie flat in front of your body. Your dominant arm starts vertically, with the elbow resting on the non-dominant hand, and your index finger extended upward while the rest of the fingers are drawn into a fist (i.e., the letter 'D' in ASL). Make the sign by sweeping your dominant hand down toward your non-dominant forearm, so your index finger traces an arc in the "sky", which is the area around your arms.
Day can be taught more easily by contrasting a picture of day with a picture of night.
There are two ways to use and teach the sign for day. Teach it to introduce concepts of chronology (10 days until Christmas) or to contrast day & night.