If you want to replenish your Apple ID credit for iTunes and App Store purchases by the 24th, you should do so now: Apple adds 15% bonus.
It is quite unusual for Apple to offer discounts directly in its own store, perhaps that’s why the promotion is so well hidden.
15% get bonus on the iPhone and iPad
On mobile devices, you can get the 15% bonus when you top up credit on the App Store. So open App Store App and go to Your profile at the top right. Then go to Your name > Add funds to Apple-ID. There you will get the possibility to add credit with the bonus. Make sure that the action is only valid once for you, so decide before which amount you want to top up.
15% get bonus on Mac
On the Mac, too, the way leads via the App Store. So open the Mac App Store App and click on Your name. Then click Your profile or Account, then select Add funds to Apple-ID.
Apple has hidden the action well and it’s only valid until the 24th. So spread the word!