Setting the timer to auto restarts after 7 days – infinitely

Hello,

I would like to create this kind of scenario:

I have a variable product (3 variables, one piece, two pieces, three pieces of product) that is always on sale, lets say 50% off all time.
I would like to show countdown timer for customers that visit my site, it is set for 7 days and after 7 days it automatically restarts for another 7 days – infinitely.

I tried to set it up with no success.
Can you guys give me a hint to to properly set it up?

All the best,

Jakub