The Celestial Tournament is a special instance and by most seen as the pinnacle of pet battle challenges.
You can do it once a week and inside, you will face three pet battle masters, followed by the four Celestials to fight against. The three masters change every week and there's a total of three sets of them rotating through.
Doesn't sound that hard, but what makes it tricky is that once you're in, you cannot heal your pets anymore! That makes planning ahead very important.