Blizzard will be performing maintenance on all its titles tomorrow morning, January 31. Battle.net will be down from 5 AM PT until 6 AM PT, and Battle.net, World of Warcraft, Hearthstone, Starcraft II, Heroes of the Storm, Diablo 3, and Overwatch will be undergoing maintenance from 7 AM PT until 10 AM PT. World of Warcraft, in particular, is marked as "New Logins Unavailable," so the services on some Blizzard titles may be sporadically available during the designated times.
Currently, a host of events are running in Blizzard's titles. Overwatch features the Year of the Rooster Chinese New Year Event, which ends February 13. The Diablo III Anniversary Event is winding down and is set to conclude at the end of this month. Additionally, the Cow Level in World of Warcraft, the Diablo III-themed Hearthstone Tavern Brawl, and the Overwatch Diablo III-themed items will likely become unavailable after tomorrow, so be sure to experience them before they're gone.
It's no surprise that Blizzard's servers require maintenance. Overwatch recently clocked over 25 million players registered, which has to take some serious computing to maintain. You can keep updated on Blizzard's server and game status on the Blizzard Customer Service official Twitter.