Nintendo has provided details on an upcoming spot of maintenance set to take place over the next couple of days. The maintenance shouldn't impact your gaming time too much, but could interfere with some online services.
Two sessions are planned this week, relating to cloud saves and the transferring of save/user data. Nintendo warns that "during the maintenance window, network services may be unavailable." We have all the times you need below:
Nintendo Switch Online - Save Data Cloud
US: 29th June, 6pm PT - 8pm PT / 29th June 9pm ET - 11pm ET
UK: 30th June, 2am - 4am
Europe: 30th June, 3am CEST - 5am CEST
Switch - Transfer Your User Data / Transfer Your Save Data
US: 29th June, 9:30pm PT - 10:30pm PT / 30th June, 12:30am ET - 1:30am ET
UK: 30th June, 5:30am - 6:30am
Europe: 30th June, 6:30am CEST - 7:30am CEST
It's worth pointing out that planned maintenance doesn't necessarily mean that any major Switch firmware updates are on the way, so don't expect to see lots of exciting changes once it's all wrapped up. Still, if you notice any hiccups with Switch online services during these times, at least you now know what's causing them.
The latest Switch firmware update arrived earlier this month: Ver. 10.0.4 (Released June 4, 2020).