You might want to avoid Korea in June/July! Hotness prime time usually starts June at the latest. Temperatures, I think, goes up I think during May usually, and it stays really hot until, let’s say, mid September I should think. Last year, the hottest temperature ever in Korea was recorded, 38.2C, so nearly 40C! But usually it stays around 30 to 35 degrees, and that’s really hot if haven’t felt it. Rarely goes down below 30s, might get a low 28C if you’re lucky. You can’t sleep if you don’t have air conditioning. At all. Temperature doesn’t go down by far during the night, so it still stays above 25C; when I went there I made the mistake of not turning the aircon on… and what a hellish night it was.
If the place you stay has air conditioning, then you’ll be fine for the night and for residence. Just wear loose and short clothes while you go out, you get used to the heat after a while. If you don’t have air conditioning in the place your staying… well, let’s say you’ll be wearing nothing but underwear like, all the time around.