These are singular points in time, not a group of things. You could use terms like:
* The two cardinal points of the day: This highlights their importance and distinction.
* The bookends of the day: This emphasizes their position as the beginning and end of the day.
* The midday and midnight hours: This is a more neutral and descriptive way to refer to them.
Ultimately, the best choice will depend on the context and the specific meaning you want to convey.