Premium-reward items are locked (indicated with a padlock) until you purchase a regional pass that lets you claim them. Each region has its own regional pass. As soon as you unlock the premium pass, all the premium rewards you've collected before will become available for you to obtain:

What happens if I don't want to buy the pass?

You can still participate, but it will take a bit longer to unlock some rewards, like the Goblin King. However, even if you don’t pay for a pass, you can still dig for free rewards!