Starbucks knows people love the word "free."

They also want customers to glug down as many Eggnog Lattes and Peppermint Mochas before the holiday season is over. 

Holiday season comes early to Starbucks

And so, Starbucks has a brilliant plan to get the holiday cheer -- and holiday buying -- started as quickly as possible.

Starbucks started serving their holiday drinks super early (Nov. 2) and is offering a free reusable holiday cup to anyone who purchases one. Bring it back all season long, and you'll get 50 cents off grande holiday drinks after 2 p.m.

Guess what. People are going crazy for the free cups. And Starbucks can't keep up. It appears they may not have anticipated the demand.

Starbucks loyalists can order their drinks through the app and earn rewards. The morning of the promotion, the Starbucks app crashed, giving users a message that it was "down for scheduled maintenance." Forbes reporter Micheline Maynard says that's not true. Starbucks apologized on Twitter for the inconvenience.

Meanwhile, in investor land, Starbucks is killing it. Shares were up as much as 12 percent on the day of the promotion, Bloomberg reported.

It's not about going green

First Starbucks announced its plan to ditch plastic straws. Now, they're trying to get more people on the reusable cup bandwagon. Of course, this isn't entirely altruistic on the part of Starbucks. They really just want you to buy more Caramel Brulée Lattes.

Last year's holiday drinks didn't sell so well. This year, they needed to find a way to pick up sales and ramp up the holiday drink guzzling.

Chief Operating Officer Rosalind Brewer admitted to CNBC that Starbucks made a reusable holiday cup for two reasons: People asked for more reusable cups, and offering the discount with the cup would help Starbucks sell more.

So maybe this year's holiday cup is just a stunt. But if this gets people to toss out fewer cups, that's a good thing. Drink up!