First, when God created the heavens and the earth, He states that the evening and the morning were the first day, etc. beginning in Genesis 1:
The Creation of Light
Genesis 1
1 In the beginning God created the heaven and the earth.
2 And the earth was without form, and void; and darkness was upon the face of the deep. And the Spirit of God moved upon the face of the waters.
3 And God said, Let there be light: and there was light.
4 And God saw the light, that it was good: and God divided the light from the darkness.
5 And God called the light Day, and the darkness he called Night. And the evening and the morning were the first day.
When we remember a day we often also mean a 24 hour period of night and day, and so it is. Even a partial day is considered a day in the life though the rememberance of an activity may be but a part of the day.