Did you know that we use the Instrumental case with the parts of the day, without any preposition? For example, утром (in the morning), днём (in the afternoon), вечером (in the evening), ночью (at night).
We also use the Instrumental case with the adjectives, numerals and pronouns used with the names of the parts of the day.
Этим утром мы завтракали в кафе. This morning we had breakfast in a cafe.
Той ночью было очень холодно. It was very cold that night.