Nintendo is rolling out a new idea to help make their Nintendo Switch Online service more appealing. Starting on August 7th, any current subscribers to NSO will be able to play Mario Tennis Aces for free, using Nintendo's newly introduced Game Trials. You'll be able to play as much of the game as you want until August 13th, in which the trial will end, and afterward, Nintendo will hold a 33% off sale for those interested in buying the game, following the week-long demo. Any progress you make in the Game Trial version of the game will be carried over to the full product, should you choose to purchase it. Currently, this deal has only been announced for the EU/UK, while Japan will be offering Captain Toad: Treasure Tracker as their first Game Trial title. There's no word on if this will be coming to North America yet.
Edit: North America will be getting this too.