I have been several times at this exact time and several of my friends and families will be there this year. I would not pack any long pants, unless one pair to wear on plane or car. Shorts will be fine. As to sleeveless my family and I always wear t-shirts with a short sleeve, too much sun too quickly if we don't.
Always pack less, you will want to buy some cute t-shirts and this way you will have an excuse to wear them.
Have fun and remember to drink lots of fluids because it will be already very hot.