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The new launcher already says this, but there is going to be a major update to Mojang's login servers tomorrow (May 10, 2017) at 8:00 AM UTC. Logins to Minecraft via the launcher will be down and logins to Minecraft servers and networks will also be down. The updates usually take 30 minutes to 2 hours, depending on certain conditions.
Some of you may be wondering when 8:00 AM UTC is, so I've included this link: https://time.is/UTC
Thanks for reading and have a great day!