https://www.gamespot.com/articles/pokemon-go-trading-event-going-on-now-for-a-limite/1100-6460971/

A new in-game event is underway in Pokemon Go. Following the recent rollout of Lucky Pokemon, developer Niantic has kicked off a special trading-focused event that nets players extra Candy, XP, and other bonuses when they trade Pokemon with friends.

The event runs from now until 9 PM PT on August 19 (12 AM ET on August 20). During that time, all trades with friends--even those who you've only recently added--will cost 25% less Stardust than they normally do. This applies to both standard and Special trades, the latter of which are for Legendary or Shiny Pokemon.