The promotion is in celebration of the company opening its 100th store, this time at the Boston Seaport. The catch? The Shack (SHAK) is only giving away free burgers to the first 100 customers at each of its locations worldwide.

The promotion won’t last all day, either. Customers can only get one free, single ShackBurger before 12 p.m., and Shake Shack locations in ballparks or stadiums won’t be included in the promotion. But good news if you want to get there early: All restaurants will be opening at 10:30 a.m. local time today instead of 11 a.m.

Let's party! In honor of 💯 Shacks, we're servin' up FREE burgers on 8/16: https://t.co/5JizPyqDWS pic.twitter.com/sEgaSf4TgE

— SHAKE SHACK (@shakeshack) August 14, 2016

Shake Shack announced the celebration last week. What started out as a small stand in Madison Square Park in 2004 has since expanded to 15 states and the District of Columbia, as well as internationally in London, Tokyo, Istanbul, Dubai, Moscow, Seoul, and more.

Shake Shake is a part of the “better fast food” movement, and is often recognized for its antibiotic and hormone-free meats, cage-free chickens, among other things.

After its initial IPO in 2015, the burger joint saw its stock price surge over 130% on the first day of trading. But the company’s shares fell 8.7% last quarter, making some worried that growth at stores that have already opened has slowed.