As of February 11, 2020, new events have a new feature: the Event Pass. Participating in the event gives you Event Points; collect these points to unlock rewards.
The event pass has two rows: regular and premium. Points can unlock both rows at the same pace, but the premium row is only available to those who pay for it for that event. All unlocked rewards must be claimed before the event expires!
The regular row generally contains energy, coins, XP, roots and herbs, food, region-appropriate materials, and gems.
The premium row contains a costume, more energy and food, green rings, very good equipment, and gems.
Additionally, buying the premium row will activate a special speed potion for the remainder of the event. If you already have a speed potion active, its time will be added, not lost.
For a list of event pass items in a specific event, see Events.
Costumes replace the old outfit system from the Flash version of the game.
Unlike outfits, costumes are only cosmetic; they don't affect digging at all, only what Diggy looks like.
Currently, two basic costumes are available for all users: Current Diggy and Retro Diggy. Current Diggy is the Diggy introduced with WebGL; Retro Diggy is from the Flash version. There is no costume (yet?) for the old Mobile Diggy.
Event costumes are the first item unlocked in the premium row of event passes; once claimed, they are available permanently.
To switch costumes, go to the Costume tab in your inventory.