https://www.gamespot.com/articles/nintendo-switch-online-update-coming-tonight-along/1100-6461836/

Nintendo's first paid online service is just hours away, but you may notice a short interruption in online service while it gets rolled out. The Switch online launch will be preceded by a period of server maintenance, after which you'll be able to jump in, sign up for the service, and play some of the classic games being offered.

The Nintendo Support page says that the 6.0 update will go up at 5 PM PT / 8 PM ET, followed by a three hour maintenance period. Once the maintenance completes, around 8 PM PT / 11 PM ET, you can log on to purchase your Nintendo Switch Online membership, but you'll need to update to version 6.0 first.