I attended session 1 in 2018, and we just wore our issued uniform of a t-shirt and blue shorts for the awards. In fact, I wore only the clothes they gave us from Sunday morning until after I left the yard. I needn't have wasted space packing pajamas because it was faster and easier to sleep in the PT gear for the next morning.
For the first day, I don't think it's necessary, but you can. After an irrationally large amount of internal debate, I wore my Harry Potter spell t-shirt and a pair of hiking shorts. One detailer told me that my shirt was pretty awesome, which helped me relax and get excited about the week. As long as you don't look like a slob, are dressed modestly, and can easily tuck in your shirt (the detailers will have you tuck it in before you enter the hall to get all of your stuff if it's not already tucked in), you'll be fine.