Every day you are offered to purchase special packs called bundles or special offers. Every 16 hours, you get a different bundle. Under the offer icon on the main game screen, the timer displays when this offer expires.
The pool of bundles is the same for every player. They are created manually, their contents and price are fixed, and their distribution is a fully automatic process. There is no limit to how many times you can be offered the same bundle. If you miss one, you will eventually see it again.
Note
When you reach VIP 10, you unlock the Сarousel of Bundles that allows you to choose a bundle for purchase. Carousel resets every 16 hours, but new bundles might be added to Carousel earlier if some bundles are sold out.
In addition, you can purchase some bundles only once. Therefore, you can buy every single bundle in the game, and your choice of bundles might become limited. However, new offers are added to the game regularly.