Many will take to Facebook tomorrow to change their profile pictures to feature a red X. But you might not be aware of the reason.

The red Xs belong to the End It Movement, dedicated to ending modern-day slavery by partnering with leading organizations throughout the world, and raising over $4 million since 2013.

To honor the movement’s “Shine a Light on Slavery Day,” members of Congress, as well as celebrities like anti-trafficking activist Ashton Kutcher, are expected to mark their profiles with a red X, according to Refinery 29.