https://www.gamespot.com/articles/nintendo-switch-online-start-date-confirmed-7-day-/1100-6461709/

Nintendo has finally confirmed that online services for Switch will be available from September 18 in America and September 19 in the UK. Additionally, people will be able to sign up for a free seven-day trial on the Eshop at launch. As previously detailed, the service gives subscribers the ability to play games online, enjoy a library of NES titles that'll be regularly updated, and back up some save data to the cloud.

Memberships can be purchased for one, three, and 12 month increments for $3.99 / £3.49, $7.99 / £6.99, and $19.99 / £17.99 respectively. These options are one per Nintendo Account holder; a family membership is also available for $34.99 / £31.49, and this allows up to seven others to benefit from the Nintendo Switch Online service.