“And there was evening and there was morning” teaches that the measure of day and the measure of night were created on the first day.
On the daf / in context: Still within the first-day inventory on Chagigah 12a, the phrase is cited as the source for the creation of the temporal measures themselves.
Type of Learning: Derashah from phrasing — first-day creation of day/night measuresWhat Is Learned: The lengths of day and night were fixed on the first day of creation.