A point in time
に can be used to indicate an absolute point in time. Such points include days of the week, months, years, calendar dates, and time of the clock.
- Every day I wake up at 8:30.
An action toward a person
に can be used to indicate when an action is directed toward someone.
- I gave a pen to my friend.
Note that the verb for receiving (もらう) may seem reversed, but the logic is sound. See the note on giving and receiving for more.