Hot events are weekly , starting from monday and ending at sunday. Rewards are given by completing various objectives, that often change depending on the week. Some common hot events are :

Recharge Pack : Recharge balens

Daily Scratch :    Scratch a specific amount of cards

Daru Alchemist : Practice alchemy

Refiner Pack :    Refine equipment

Gemming it up : Synthesize lvl 5 or higher gems

Astro Pack :      Practice astro

Spelunker Pack : Clear catacombs or necropolis

Online Pack  :   Stay online for a specific time