If your launcher states “World of Warcraft is up to date. Downloading data for the next update." please allow the Background Downloader to continue downloading the 4.3 data. You can ensure it continues even when not playing the game by going to the launcher Options, Downloader Preferences, and checking the box labeled “Download after exiting World of Warcraft”. If your launcher only states “World of Warcraft is up to date” then you’ve already obtained the 4.3 pre-download data and will only need a small update before logging in.
For more information on 4.3: Hour of Twilight please visit: http://us.battle.net/wow/en/blog/3964559/
Please be aware that while Season 10 will be ending with the release of 4.3, Season 11 will not begin until a future date. More information about the start of Season 11 will be posted on WorldofWarcraft.com in the coming days.