Nintendo Switch Online launches tonight, and Nintendo Switch’s 6.00 firmware update also goes live then. However, before then, the eShop will be going down for some scheduled maintenance, Nintendo has announced on its support website.
The scheduled maintenance will be from 5PM PST to 8PM PST (which translates to 8PM EST to 11PM EST); the update itself is scheduled to go live once the eShop returns from maintenance. Presumably, this maintenance is the final bit of housework Nintendo is doing to prepare its servers and its system for the new functionality they will have to support going forward.
As for Nintendo Switch Online itself, it will cost $20 a year, include free NES games enhanced with HD resolutions and online play on an ongoing basis, be necessary for online play on Switch (except with free to play games like Fortnite), and have cloud saving (though not for all games). Nintendo Switch’s firmware update 6.00, meanwhile, doesn’t add all that much, mostly sticking to laying the groundwork for Nintendo Switch Online.