What to Wear in 70 degree weather?

I'm going to Naadam(wiki it) and it's going to be 70 degrees that day. My friend wants me to wear shorts, and a tee. my mom says to wear something warm though, because last year it was the same 70 degrees, but it was cold though. I thought I could wear shorts and leggings underneath, but I'm worried il get chilly(I get sick easily). Are there any suggestions of what to wear in this kind of weather? I can tell my friend tom see if she has them too(we're going to match clothes)