In an announcement that "startled the television world," Fox says it is moving "American Idol" — the biggest reality show on TV — to Wednesday and Thursday nights this winter. "Idol's" weekly performance and result shows have respectively anchored the network's prime-time line-up on Tuesdays and Wednesdays for years. The shake-up is designed to let Fox get a lock on Thursdays, the most lucrative night for selling ads. Why is Fox making the switch? (Watch the new judges' first appearance together)

It's got Tuesdays protected: The Tuesday edition of "American Idol" was the perfect lead-in to hook viewers on Fox's musical comedy "Glee," says Alan Sepinwall at Hit Fix. But "Glee" is now a "very profitable hit" and no longer needs the help, freeing up Fox to move "Idol." And Thursday night is key because movie studios use it to advertise their weekend releases.
"Analyzing Fox's mid-season schedule shake-up"

Thursday will be an easy win for Fox: The network is struggling, but it sees an opening "at a time when the other networks have lost steam on Thursday nights," says MediaBuyerPlanner. "The truth is, even if the show sinks by more than a third in ratings, it will still be the top show on TV and would handily win Thursday nights."
"'American Idol's' shift to Thursdays should ensure win for Fox"

Actually, Fox is playing defense: "With Simon Cowell — arguably the show's biggest draw — gone this season," the show's ratings are bound to "slide," says Lisa de Moraes at The Washington Post. By moving results night to Thursdays, "the most competitive night of the week," the "Fox suits" are just preparing to blame any ratings drop on the scheduling switch instead of the show's dwindling appeal.
"Fox finally moves 'American Idol' to Thursdays"