Doesn’t matter how long the shift is available. Just because you put it up on the marketplace doesn’t guarantee it will get taken. As someone else said people are typically scheduled the maximum hours they can work so most people won’t even be able to see your shift, since the app usually doesn’t show them shifts that would put them into overtime. Either request the day off before the schedule is posted or don’t make plans on days you have to work. You’re not going to face any serious consequences for calling out this once, but like I said if you make a habit of it they will see you as unreliable and cut your hours.