Unfortunately, Riot Games doesn't reveal when the next Night Market will be until at most a week beforehand. Since they only happen about six times a year, you'll want to jump on the opportunity to visit it while you can for your chance to get some exclusive skins for a hefty discount.

In total, there have been 11 different Night Markets since the game was released. Their dates are listed below:

As you can see, the Night Market tends to happen at around the same times every year, but it only runs for a couple of weeks at a time — so you'll want to be sure you log into the game during that time to see what skins have been made available to you.

Even if you don't see the announcement from Riot Games before the Night Market begins, you should receive a notification that it's happening when you log into the game. Follow the prompts on the menu to access it and make your purchases.

Once the prompt for the Night Market disappears, you can access your selections again by clicking on the card located in the top-right corner of your screen, right next to the store tab (so long as the Night Market is still happening, of course).

You don't have to buy all of the skins available to you at the Night Market, but if you don't make your purchases before it ends, you will not have another opportunity to buy those skins at the discounted price unless they appear again at the next market.