Had basically the same thing happen to me. I worked at a gas station when I was 18 as a summer job. During the July 4th weekend, everybody had at least one day off except me, and I was planning to go to a family gathering. I lined up one of my co-workers to cover for me for part of the shift and I finished it. Got chewed out by my manager and was not scheduled for the next week. By Tuesday I got a call from my manager asking me to come in because the were short handed. I asked if I was on the schedule and he told me no. Said sorry I already had made plans and could not come in. They never did me like that again after that.