You can do an online search that will give you the average range of temps and then keep an eye on the accuweather forecast as your trip approaches. We went last Dec and are returning this year (34 days to go!!), and here's what we packed and wore:
shorts
t-shirts
jeans
long-sleeved shirts
"microfiber" fleece -- it's thin polar fleece
swim suits
medium-weight winter jackets, gloves, hats
Day by day, and more importantly mid-day to evening, the temps changed a lot. Wear things you can layer, avoid packing bulky sweatshirts if you can (or just plan to buy them) and think about running a load of laundry while you're there. If you're doing anything at night, be prepared with hats and gloves. Finally, for long days, pack up extra clothing in a backpack and stow it in a locker in the morning so you have it without having to lug everything around all day.