Would like to know what the wather in Seoul is going to be like the end of September and beginning of October? I have read it is the best time to come as it does not rain much. However, I have also seen the temperature charts that say that the evening temps drop significantly, is that right? What do I need to bring to wear during this time of year?
Depends what you are doing. Are you going hiking in the surrounding mountains or sightseeing in the city? What do you normally do when you travel? October is the season after the typhoons have swept the monsoon away for another year. The mountains are full of colour and very rarely there will be a sprinkling of snow on the higher peaks. The first snowfall usually comes about mid-November. It is mild in the daytime and bites a bit after sunset, but the nightlife should take the edge off the chill....
In general, October is the end of shorts season. Long pants, long-sleeved shirts, light jacket at night. That sort of thing. Some of the weather predictions for the short near-term in Seoul have suggested possibley cold nights, but it is very possible that it could warm back up, both during the days and nights.

Hi guys, Thanks for the comments.
Mostly we will be in the city, but we are hoping to travel up to the DMZ, and if that doesn't work out, perhaps a trip into the mountains if the trees are turning when we go.
So in the day time a long sleeve shirt without jacket is enough? and at night we only need a light jacket? But it seems the temperature is like 4 degrees at night it appears a winter jacket would be needed no? If we stay out past 10pm I suspect it will be quite cold no?