Why is the Seoul Fireworks Festival being held so early this year???

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I thought it was strange after reading a post saying it's happening tomorrow.
It's always been cold when I went to see it before,
but it's not cold right now, so why??
I searched for it and found that it's usually held in early October, except for the year before last when it was in late September.
This is the first time I've seen it held in early September. Why is that?