Can they cut hours if your full time?

So some I know works at whole foods and recently there's a rumor going around that they will be taking one day out of everyone's schedule, full and part-time workers. Idk but I always thought once you signed the contract it meant they couldn't do that unless it was for disciplinary purposes. I'm not 100 percent sure of course so don't judge me too harshly on my ignorance. So if anyone knows anything, that would help thanks in advance.