As a freelancer, if you work on-call, you'll work whatever hours are needed to fill in -- if someone calls in sick, or someone leaves, or if they just need extra help. Generally, you might work on-call for closing shifts, because they tend to be busier. If you freelance for an event, there are certain hours when freelancers come in. For permanent folks, in retail in general, those with more seniority usually have more say in what hours/shifts they work...but you figure, there's gotta be someone covering every shift, and more during peak hours. I don't know if that helps anyway...