By Matt Bemer
Updated: August 15, 2016 1:47 PM ET | Originally published: August 12, 2016

To celebrate the opening of its 100th location at the Boston Seaport, Shake Shack is giving away a free single ShackBurger to the first 100 guests at any of its locations worldwide on Tuesday, August 16.

All U.S. Shake Shack restaurants will open at 10:30 AM to welcome freebie-loving guests. The promotion ends at 12 noon and excludes Shake Shack locations in ballparks and stadiums.

The 550-calorie ShackBurger consists of a 4-oz. patty of hormone- and antibiotic-free Angus beef, lettuce, tomato and some ShackSauce, all on a potato bun. The regular price is $5.29.

Bad news for vegetarians, though: the promotion is only good for a burger of equal or lesser value. Shake Shack’s veggie option, the ‘Shroom Burger, costs a buck more than the ShackBurger.

Shake Shack’s CEO Randy Garutti called the opening of the company’s 100th location an “epic milestone for us” in a press release on Thursday. The announcement comes after a tough week for the company. Shares of Shake Shack tumbled Wednesday after the millennial-friendly burger chain reported a dip in quarterly sales due in part to increased competition from grocery stores.

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