Nintendo announced through Twitter that Nintendo Switch Online will launch on September 2018.
Nintendo first announced that the company would be introducing an online service back along with the Switch announcement. However, the online service ended up being pushed back from its 2017 release window into 2018. Now after much delay, we will finally see Nintendo's online service this September.
Nintendo Switch Online will feature services like online lobbies and voice chat, along with a "classic game selection," and Nintendo eShop deals. The classic games are reported to be a monthly free game subscription style service where online users will have access to an ongoing library of classic game each month. So far only NES games are part of the classic games selection.
The online service subscription will be rolled into the Nintendo mobile app, which so far only supports a limited number of games like Splatoon 2. No word yet on any updates for the app.
Nintendo's online service is currently priced at $3.99 for one month, $7.99 for three months, and $19.99 a year. Far cheaper than any other online service for other platforms.