I don't believe "seasonal hires" start until mid to late October but contact your steward to be sure. (If you dont know who he is, ask the guy who works beside you.) You can also ask the FT preload supervisor (probably the one you interviewed with, perhaps along with the HR person who contacted you). This is obviously a very important distinction because if you are considered seasonal then the next three months will be washed down the drain as it relates to your seniority, raises, etc.
When you filled out your app both online (perhaps in person as well) did it spell out for you that this was seasonal? If not you should be good. Most likely you are now under a 30 day "training period," show up on time, put up with the unrealistic production expectations as best you can and keep your misloads low-to-zero and you'll be fine, more or less.