Dunkin' Donuts (NASDAQ: DNKN) may have slimmed down its standard slate of doughnut offerings, but it's sticking to its long-term habit of rolling out limited-time menu items around the seasons and holidays. And for St. Patrick's Day, it's not only adding some special tastes, it has also launched a contest.

What is Dunkin' doing?

The coffee and doughnuts chain has asked its customers to tweet about how they'll celebrate St. Patrick's Day with the hashtags #DDLuckyDozen and #Sweepstakes. A dozen winners picked from among those who do will receive a year's supply of doughnuts, and one grand prize winner will get a chance to visit the company's headquarters in Boston and create his or her own doughnut flavor at its test kitchen.

People can enter the contest on Twitter and Instagram through St. Patrick's Day (March 17). You must be 18 to enter; full rules can be found at DDSweeps.com.

For those more interested in eating than tweeting, Dunkin' has temporarily added a green-frosted Mint Brownie doughnut to its menu, along with a Girl Scout cookie Thin Mint-flavored Frozen Chocolate drink. The Mint Brownie doughnut is filled with mint chocolate brownie batter-flavored butter creme, topped with light green dipping icing sprinkled with brownie crumbles. Some Dunkin' locations will also offer additional doughnut varieties finished with four-leaf clovers or green drizzle.

Why is Dunkin' doing this?

Limited-time offers can lead to bigger checks as regular customers who might usually only order a basic coffee add a special doughnut or try a more expensive drink. In addition, these promotions can entice lapsed customers and even loyalists of other chains to visit a Dunkin' Donuts.

And of course, such offers create a sense of urgency for consumers, who know they only have a short time to try the products. Meanwhile, the social-media-hosted contest should help the chain raise awareness about its holiday offerings.

Whether these particular special menu items find favor or not almost doesn't matter. Over the course of the year, the chain will throw a lot of promotions against the wall. Some will work, and be brought back in subsequent years; others won't.

But over time, the chain will develop a slate of seasonal offers its fans will anticipate, and a set of deals it knows will reliably spike sales during certain periods. Though for every highly successful offering a restaurant chain debuts, there are usually a few that do little to move the needle, such a strategy is well worth pursuing.

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