Since you specified best UI, unless you meant UX, I will try to give an example that not only works well but looks good too. A natural language form would be an good example of this.
Demo Here: http://tympanus.net/Tutorials/NaturalLanguageForm/
For this you can turn the expression into a natural sounding sentence so it is more easily understood and looks good. The underlined elements are dropdown choices that allow you to easily set up your expression. Yours could read something like:
Set me a reminder for every Friday at 7:00pm.
every being a dropdown of: this/every
Friday being a dropdown of: Monday/Tuesday/.../Friday
7:00pm being a dropdown of: 12:00pm/.../12:00am