First, as HLM just posted, you should have mentioned this during the hiring process (Although, last year at my store, I know people did this and management just scheduled them anyway. Three good seasonals left because of it.)
Second, it is after Christmas so we aren't really that busy. So, they should be able to accommodate you.
Third, the past few years we started getting rid of seasonals right after Christmas, The situation may take care of itself. You may be let go before those dates.
In any event, go talk to your ETL and explain the situation. If they say OK, put in for the days off through Workbench (ask your HRTM how to do it). Then keep checking to see if it was approved. If after a couple of weeks it hasn't been, remind your ETL. Do not assume they will have approved it, because if they don't approve it in a certain amount of time, it will be auto-denied.