The Switch will be undergoing some maintenance to its online services tonight. This will result in interruptions to some of the network services, such as online multiplayer.
The network will be undergoing maintenance for about 2.5 hours, although it’s likely that some online services will be down for less than that.
Nintendo didn’t go into detail as to what’s updated, but it seems that it’s the usual back-end server updates that have to be done once in a while.